Document LCB_SERVER_BUG and LCB_PLUGIN_VERSION_MISMATCH. Enhance the the lcb_strerror test to detect undocumented error codes.

Commands sent to multiple servers fail to detect the respose if mixed with other commands.

Issues: CCBC-150

Under high load the library could generate LCB_ETIMEDOUT errors without reason owing to internal limitations.

Issues: CCBC-153

Cancellation of the HTTP request might lead to memory leaks or to segfaults (2e3875c2).

Issues: CCBC-151