lib: provide _notmuch_database_log_append

_notmuch_database_log clears the log buffer each time. Rather than
introducing more complicated semantics about for this function, provide
a second function that does not clear the buffer. This is mainly a
convenience function for callers constructing complex or multi-line log
messages.

The changes to query.cc are to make sure that the common code path of
the new function is tested.