Fix GetNewTransactionId()'s interaction with xidVacLimit.

Fix GetNewTransactionId()'s interaction with xidVacLimit.

Commit ad308058 switched to returning a FullTransactionId, but
failed to load the potentially updated value in the case where
xidVacLimit is reached and we release and reacquire the lock.
Repair, closing bug #15727.

While reviewing that commit, also fix the size computation used
by EstimateTransactionStateSize() and switch to the mul_size()
macro traditionally used in such expressions.

Author: Thomas Munro
Reported-by: Roman Zharkov
Discussion: https://postgr.es/m/15727-0be246e7d852d229%40postgresql.org