lib: write talloc report in notmuch_database_destroy

lib: write talloc report in notmuch_database_destroy

Since most memory allocation is (ultimately) in the talloc context
defined by a notmuch_database_t pointer, this gives a more complete
view of memory still allocated at program shutdown.

We also change the talloc report in notmuch.c to mode "a" to avoid
clobbering the newly reported log.