vinyl: add a special read view for upsert squash process.

The upsert squash process must not touch or take into consideration
non-committed statemens because they are owned by TX manager and
thus might be changed or removed by it's will.
Add a committed read view in TX manager (in addition to global
read view) that allows to see only committed statements.
Use the read view in squash process.
Move upsert count calculation, simple inplace upsert
squash and upsert process invocation from preparation stage to
commit stage of TX; without this change after a squash process
prepared statements will not get a change to use simple inplace
upsert squash.

fix gh-2382.