'database.cc' is becoming a monster, and it's hard to follow what the
various static functions are used for. It turns out that about 1/3 of
this file notmuch_database_add_message and helper functions not used
by any other function. This commit isolates this code into it's own
file.
Some side effects of this refactoring:
- find_doc_ids becomes the non-static (but still private)
_notmuch_database_find_doc_ids
- a few instances of 'string' have 'std::' prepended, avoiding the
need for 'using namespace std;' in the new file.
