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>