Bug 13606: Upgrade sessions.a_session to MEDIUMTEXT

Bug 13606: Upgrade sessions.a_session to MEDIUMTEXT

The previous patch fixed partially the problem.
If you try to update more that 1000 records, you will get the same
error.

This patch change the type of sessions.a_session from TEXT (64kb) MEDIUMTEXT (32mb).

Test plan:
1/ Try to modify a huge amount of records (1000 should be enough)
2/ Confirm it worked!

Signed-off-by: Frederic Demians <f.demians@tamil.fr>
I confirm that with more than 1000 records the session object is truncated, and
then can't be anymore be desirialized, blocking completely Koha. Fixed with
this patch.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
This only pushes off the problem expontentially (64KB vs. 16MB), but this is a
good start.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>