Initialize and Shutdown sqlite3 globals

sqlite3 recommends that sqlite3_initialize be called when the
application starts, and sqlite3_shutdown when it stops. Since we don't
always use sqlite3, we initialize it when a SQLiteDatabse is constructed
(calling sqlite3_initialize after initialized is a no-op). We call
sqlite3_shutdown when we see that there are no databases opened. The
number of open databases is tracked by an atomic g_dbs_open.

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