lib: isolate n_d_add_message and helper functions into own file

'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.