NRG (2.11): Start catchup from `n.commit` & fix AppendEntry is stored at `seq=ae.pindex+1`

[MauriceVanVeen]

This PR makes three complementary fixes to the way how catchup and truncating is handled. Specifically:

• when doing n.loadEntry(index) we need to pass where the AppendEntry is in terms of stream sequence, this is equal to ae.pindex+1 since the ae.pindex is the value before it's stored in the stream.
• start catchup from n.commit, we could have messages past our commit that have been invalidated and need to be truncated since there was a switch between leaders
• because we catchup from n.commit, we check if our local AppendEntry matches terms with the incoming AppendEntry, we only need to truncate if the terms don't match

Signed-off-by: Maurice van Veen github@mauricevanveen.com

[derekcollison]

LMK when ready for review.