If there is no terminating zero within the 16 magic bytes, the buffer would be
over-read in the std::string constructor. Fixed by using the "from buffer"
variant of the ctor (that also takes a size) rather than the "from c-string"
variant.
Rewrite uses the VACUUM command which does exactly what we want. A
specific advertised use case is to compact a database and ensure that
any deleted data is actually deleted.
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.
