Add Blocktree api to get transactions by slot

[CriesofCarrots]

Problem

Our main downstream use case only needs to introspect confirmed blocks, with an empty result from unconfirmed ones. Currently, blocktree offers no api to return transactions in a slot.

Summary of Changes

• Adds blocktree api get_confirmed_block_transactions. Method checks whether block is confirmed (slot is_root()) first; if so, it flattens the slot entries vec into a vec of transactions to return.
  It may make more sense to return an Iterator from this method in order to map each Transaction to its Result, once that data is available in blocktree. Will reassess when those apis are implemented.
• Updates getConfirmedBlock rpc to use now blocktree api. If get_confirmed_block_transactions returns an error, rpc method returns an empty result. JsonRpcRequestProcessor::get_confirmed_block() generates mock transactions statuses, since persistent status cache is not yet implemented (CC: feat: rocksdb (persistent) transaction status cache #6958 )

Toward #6867
FYI: @sunnygleason

[mvines]

lgtm!

[codecov]

Codecov Report

Merging #6966 into master will increase coverage by 13.4%.
The diff coverage is 71.4%.

@@            Coverage Diff            @@
##           master   #6966      +/-   ##
=========================================
+ Coverage    65.5%   78.9%   +13.4%     
=========================================
  Files         221     221              
  Lines       51355   42673    -8682     
=========================================
+ Hits        33666   33698      +32     
+ Misses      17689    8975    -8714