[lldb/test] Refactor socket_packet_pump

Now that the class does not use a thread, the name is no longer
appropriate. Rename the class to "Server" and make it a long-lived
object (instead of recreating it for every expect_gdbremote_sequence
call). The idea is to make this class a wrapper for all communication
with debug/lldb-server. This will enable some additional cleanups as we
had some duplication between socket_pump non-pump code paths.

Also squeeze in some small improvements:
- use python-level timeouts on sockets instead of the manual select
  calls
- use byte arrays instead of strings when working with raw packets