libgit2: remove connection caching

Connection caching was a feature created to resolve
upstream issues raised from concurrent ssh connections.
Some scenarios were based on multiple key exchange
operations happening at the same time.

This PR removes the connection caching, and instead:
- Services Session.StdoutPipe() as soon as possible,
  as it is a known source of blocking SSH connections.
- Reuse SSH connection within the same subtransport,
  eliminating the need for new handshakes when talking
  with the same server.
- Simplifies the entire transport logic for better
  maintainability.

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

Author: Paulo Gomes

go.mod

@@ -42,6 +42,7 @@ require (
 	github.com/prometheus/client_golang v1.12.1
 	github.com/spf13/pflag v1.0.5
 	golang.org/x/crypto v0.0.0-20220427172511-eb4f295cb31f
+	golang.org/x/net v0.0.0-20220425223048-2871e0cb64e4
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 	google.golang.org/api v0.77.0
 	gotest.tools v2.2.0+incompatible
@@ -200,7 +201,6 @@ require (
 	go.uber.org/atomic v1.7.0 // indirect
 	go.uber.org/multierr v1.6.0 // indirect
 	go.uber.org/zap v1.21.0 // indirect
-	golang.org/x/net v0.0.0-20220425223048-2871e0cb64e4 // indirect
 	golang.org/x/oauth2 v0.0.0-20220411215720-9780585627b5 // indirect
 	golang.org/x/sys v0.0.0-20220503163025-988cb79eb6c6 // indirect
 	golang.org/x/term v0.0.0-20210927222741-03fcf44c2211 // indirect