Allow some basic client commands before AOF data loads
Allow some basic client commands before AOF data loads

This commit accepts incoming connections even before the AOF
dataset has been loaded into memory. Though only a very limited
command set is allowed.

Allowed commands:
  PING, ECHO, OUTPUT, QUIT

All other commands will return:
  LOADING Tile38 is loading the dataset in memory

This is useful for establishing connections for the purpose of
checking process and network state.

tidwall/tile38,tidwall/tile38