CDRIVER-4193 Ensure OP_MSG for handshakes and fix RPC op_egress counters (#1256)

* Move SHM count function into mongoc-counters-private.h

* Declare test_framework_has_compressors in test-libmongoc.h

* Allow /counters/op_msg and /counters/op_compressed with auth

* Add _mongoc_rpc_op_egress_inc

* CDRIVER-4193 RTT thread should also use hello if serverApi is given

* Do not increment RPC op_egress counters in mock server

* Add /counters/rpc/op_egress tests

* CDRIVER-4121 Add /counters/rpc/egress/auth

* CDRIVER-4622 RPC op_egress increment: mongoc_async_cmd_new -> mongoc_async_cmd_phase_send

* Add mongoc_<object>_uses_loadbalanced

* CDRIVER-4265 Use OP_MSG for handshakes when loadBalanced=true

* Clarify meaning of RPC op_egress counters in documentation