libbinder: RPC disambiguate server/client

Now:
- server: RpcServer
- client: a client of an RpcServer
- incoming: a thread processing commands (either as part of an
  RpcServer's sessions or back to a client which has a threadpool)
- outgoing: a thread for sending commands (either to an RpcServer's
  sessions or back to a client which has a threadpool)

Bug: 167966510
Test: binderRpcTest
Change-Id: Iea286ab0ff6f9fb775994247003b8d29c999e10a