tests: test reconnection after server failure
omcache: fixes for status reporting plus some cleanups

Allow omcache_io to be called with request objects but no values to store
them into to make it possible to really use the response callbacks without
having to also read all values in the io() call.

Removed unused port argument from internal omc_srv_init() function

Report proper errors in omcache_command_status if servers fail.
tests: more tests for settings, multi gets, utils, etc

Test request_id wraparound, request buffering, messing with maximum buffer
sizes for response and send buffers, response callbacks, invalid server
lists, strerror descriptions, etc.

This commit brings code coverage (according to gcov) to:

File 'omcache.c'
Lines executed:95.17% of 704
Branches executed:94.95% of 713
Taken at least once:73.21% of 713
Calls executed:83.48% of 224

File 'commands.c'
Lines executed:100.00% of 76
Branches executed:100.00% of 60
Taken at least once:73.33% of 60
Calls executed:100.00% of 13