[RFC] Split notmuch_database_close into two functions

[Justus Winter]

I propose to split the function notmuch_database_close into
notmuch_database_close and notmuch_database_destroy so that long
running processes like alot can close the database while still using
data obtained from queries to that database.

I've updated the tools, the go, ruby and python bindings to use
notmuch_database_destroy instead of notmuch_database_close to destroy
database objects.

Cheers,
Justus