WIP: refactor FutureDriver

[niklasad1]

refactor-future-driver: this PR
remove-future-driver: #1070
master == master
remove-future-driver-tokio-spawn: #1080

Benchmarks:

One client with concurrent round trips

group                                            master                                 na-refactor-future-driver              na-remove-future-driver                na-remove-future-driver-tokio-spawn
-----                                            ------                                 -------------------------              -----------------------                -----------------------------------
async/ws_concurrent_conn_calls/fast_call/2       1.40   193.9±22.74µs        ? ?/sec    1.31   182.5±29.05µs        ? ?/sec    1.13   157.2±23.14µs        ? ?/sec    1.00   139.0±24.39µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/4       1.28   222.4±22.05µs        ? ?/sec    1.10   192.0±29.74µs        ? ?/sec    1.08   188.3±14.82µs        ? ?/sec    1.00   173.9±10.57µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/8       1.12   243.7±38.90µs        ? ?/sec    1.02    223.3±9.31µs        ? ?/sec    1.06   231.9±20.89µs        ? ?/sec    1.00    218.5±6.24µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/16      1.01    298.9±8.75µs        ? ?/sec    1.00    294.8±8.65µs        ? ?/sec    1.02   300.0±11.53µs        ? ?/sec    1.08    317.1±7.23µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/32      1.00   501.0±14.89µs        ? ?/sec    1.00   501.7±11.06µs        ? ?/sec    1.03   515.1±12.77µs        ? ?/sec    1.20   602.5±11.27µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/64      1.00   901.3±17.19µs        ? ?/sec    1.03   925.2±29.25µs        ? ?/sec    1.04   934.5±25.11µs        ? ?/sec    1.22  1102.3±24.21µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/128     1.00  1689.9±33.30µs        ? ?/sec    1.00  1695.4±26.92µs        ? ?/sec    1.00  1691.0±24.70µs        ? ?/sec    1.15  1942.7±33.94µs        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/256     1.01      3.3±0.09ms        ? ?/sec    1.01      3.4±0.07ms        ? ?/sec    1.00      3.3±0.05ms        ? ?/sec    1.13      3.8±0.07ms        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/512     1.03      6.8±0.14ms        ? ?/sec    1.02      6.8±0.13ms        ? ?/sec    1.00      6.6±0.12ms        ? ?/sec    1.13      7.5±0.13ms        ? ?/sec
async/ws_concurrent_conn_calls/fast_call/1024    1.02     14.1±0.28ms        ? ?/sec    1.03     14.2±0.44ms        ? ?/sec    1.00     13.8±0.26ms        ? ?/sec    1.10     15.2±0.26ms        ? ?/sec

Concurrent connections(2-1024) with 10 calls per connection (..)

group                                            master                                 na-refactor-future-driver              na-remove-future-driver                na-remove-future-driver-tokio-spawn
-----                                            ------                                 -------------------------              -----------------------                -----------------------------------
async/ws_round_trip/3                            1.53      2.6±0.09ms        ? ?/sec    1.93      3.3±0.18ms        ? ?/sec    1.63      2.8±0.22ms        ? ?/sec    1.00  1724.7±266.91µs        ? ?/sec
async/ws_round_trip/9                            2.03      8.6±1.03ms        ? ?/sec    1.90      8.1±0.97ms        ? ?/sec    1.97      8.4±0.37ms        ? ?/sec    1.00      4.3±0.58ms        ? ?/sec
async/ws_round_trip/27                           2.52     36.4±1.44ms        ? ?/sec    2.16     31.2±4.07ms        ? ?/sec    2.08     30.1±1.66ms        ? ?/sec    1.00     14.4±1.83ms        ? ?/sec
async/ws_round_trip/81                           2.14     85.8±4.17ms        ? ?/sec    2.11     84.6±2.44ms        ? ?/sec    2.20     88.4±6.40ms        ? ?/sec    1.00     40.1±4.92ms        ? ?/sec
async/ws_round_trip/243                          2.24   211.8±18.61ms        ? ?/sec    1.87   177.4±17.57ms        ? ?/sec    1.63    154.0±9.42ms        ? ?/sec    1.00     94.7±8.92ms        ? ?/sec
async/ws_round_trip/729                          2.12   544.1±30.87ms        ? ?/sec    1.86   475.9±21.03ms        ? ?/sec    1.72   439.6±25.83ms        ? ?/sec    1.00   256.3±19.02ms        ? ?/sec