v3.0.0.rc2

Please read the rc1 release for the greater changeset.

• Add support for assigning a symbol to a string or bytes field. [#181, @abrandoned]
• Add first_alive_load_balance option to rpc server if using ZMQ. Pass PB_FIRST_ALIVE_LOAD_BALANCE
  as an env variable to the client process to ensure the client asks the server
  if it is alive (able to server requests to clients). [#183, @abrandoned]