Split notmuch_database_close into two functions

Formerly notmuch_database_close closed the xapian database and
destroyed the talloc structure associated with the notmuch database
object. Split notmuch_database_close into notmuch_database_close and
notmuch_database_destroy.

This makes it possible for long running programs to close the xapian
database and thus release the lock associated with it without
destroying the data structures obtained from it.

This also makes the api more consistent since every other data
structure has a destructor function.

The comments in notmuch.h are a courtesy of Austin Clements.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>