vinyl: maintain list of all prepared transactions

This commit replaces vy_tx_manager::last_prepared_tx with a list of all
prepared transactions sorted by PSN. We need this list to find the
newest prepared transaction that is older than a given PSN, which is
required to switch to the most recent read view in the 'best-effort'
isolation level when an unconfirmed (prepared) statement is skipped.

Needed for #5522

NO_DOC=internal
NO_TEST=internal
NO_CHANGELOG=internal