omcache: bug fixes for omcache_flush_all and omcache_set_recv_buffer_max_size

Flush function now requires a server index, it didn't use either a server
index or key so it didn't work at all previously.

Fixed omcache_set_recv_buffer_max_size to set receive buffer size instead of
send buffer size.

omcache_set_response_callback was missing from symbol.map, added it there.
omcache: documentation for all public functions and some renaming

Renamed log callback function setting function and the log and response
callback typedefs for consistency.

Added and updated documentation for all public functions.