[#64570338] Fix session id bug.

Session id bug seems to appear in a multithreading environment when
another thread reuse an open connection and reset the current_session_id
useb by a previous thread.

This commit fix the bug by setting again the current_session_id at the
beginning of a transaction. Hopefully, a SQL connection shouldn't be
reused by Sequel in the middle of a transaction.
Merge pull request #229 from maxigit/64570338_session_id_not_found_

[#64570338] Fix session id bug.