Fix perf regression due to DML HA

We added checks via timescale#4846 to handle DML HA when replication factor is
greater than 1 and a datanode is down. Since each insert can go to a
different chunk with a different set of datanodes, we added checks
on every insert to check if DNs are unavailable. This increased CPU
consumption on the AN leading to a performance regression for RF > 1
code paths.

This patch fixes this regression. We now track if any DN is marked as
unavailable at the start of the transaction and use that information to
reduce unnecessary checks for each inserted row.

Fix perf regression due to DML HA

We added checks via timescale#4846 to handle DML HA when replication factor is
greater than 1 and a datanode is down. Since each insert can go to a
different chunk with a different set of datanodes, we added checks
on every insert to check if DNs are unavailable. This increased CPU
consumption on the AN leading to a performance regression for RF > 1
code paths.

This patch fixes this regression. We now track if any DN is marked as
unavailable at the start of the transaction and use that information to
reduce unnecessary checks for each inserted row.

Fix postgres version assert in telemetry

The telemetry code that reads the postgres version was not updated
when support for older postgres version was dropped so we introduce
a new macro PG_MAJOR_MIN which is the oldest pg major version we
support.