libbinder: RPC big 'session' rename

Before, 'connection' was used for a group of TCP connection. This
updates the names for most types to work with the following definitions:

- session - group of connections
- connection - a single tcp connection
- socket - only when referring to a socket being setup for a connection

Bug: N/A
Test: binderRpcTest, binderRpcBenchmark
Change-Id: If07afd9af680cd2a5ece6506df5383e5cc258663