lib: call g_mime_init() from notmuch_database_open()

As reported in
id:"CAEbOPGyuHnz4BPtDutnTPUHcP3eYcRCRkXhYoJR43RUMw671+g@mail.gmail.com"
sometimes gmime tries to access a NULL pointer, e.g. g_mime_iconv_open()
tries to access iconv_cache that is NULL if g_mime_init() is not called.
This causes notmuch to segfault when calling gmime functions.

Calling g_mime_init() initializes iconv_cache and others variables needed
by gmime, making sure they are initialized when notmuch calls gmime
functions.

Test marked fix by db.