Improve turba API error logging
We really don't need to save the $lastuntil value.
Allow option to ignore $timestamp when validating the syncCache.

Prevents PING requests from exiting prematurely in certain cases.
Merge changes from FW_5_0.
Merge branch 'improved-turba-API-logging' of https://github.com/thomasjfox/horde into thomasjfox-improved-turba-API-logging