Update replication statistics after every stream/spill.

Update replication statistics after every stream/spill.

Currently, replication slot statistics are updated at prepare, commit, and
rollback. Now, if the transaction is interrupted the stats might not get
updated. Fixed this by updating replication statistics after every
stream/spill.

In passing update the docs to change the description of some of the slot
stats.

Author: Vignesh C, Sawada Masahiko
Reviewed-by: Amit Kapila
Discussion: http://postgr.es/m/20210319185247.ldebgpdaxsowiflw@alap3.anarazel.de