lib: Wrap notmuch_database_add_message in an atomic section.

lib: Wrap notmuch_database_add_message in an atomic section.

Adding a message may involve changes to multiple database documents,
and thus needs to be done in a transaction.  This makes add_message
(and, I think, the whole library) atomicity-safe: library callers only
needs to use atomic sections if they needs atomicity across multiple
library calls.