fix: Sqlx SQLite expiring sessions at wrong time

[kroucher]

Fixes #9

Implemented fix as per suggestion, casting both the column value as well as the comparison value as datetime.

All tests passing, manual tests show the issue reported in #9 is no longer happening (sessions are correctly being removed on time).

Also fixes import locations in documentation to allow for all tests to pass:

-     /// use tower_sessions_sqlx::{sqlx::MySqlPool, MySqlStore};
+     /// use tower_sessions_sqlx_store::{sqlx::MySqlPool, MySqlStore};