Connection slots reserved for replication

Hello hackers,

at the moment it is possible to reserve some amount of connection slots for
superusers and this behavior is controlled by
superuser_reserved_connections configuration parameter with the default
value = 3.

In case if all non-reserved connection slots are busy, replica fails to
open a new connection and start streaming from the primary. Such behavior
is very bad if you want to run postgresql HA clusters

Initially, replication connections required superuser privileges (in 9.0)
and therefore they were deliberately excluded from
superuser_reserved_connections.
Basically that means it has never been possible to reserve come connection
slots for replication connections.

Later (9.1) it became possible to create a user with REPLICATION and
NOSUPERUSER options, but comment in the postinit.c still tells that
superuser is required.

Now I think now it is a time to go further, and we should make it possible
to reserve some connection slots for replication in a manner similar to
superuser connections.

How should it work:
1. If we know that we got the replication connection, we just should make
sure that there are at least superuser_reserved_connections free connection
slots are available.
2. If we know that this is neither superuser nor replication connection, we
should check that there are at least (superuser_reserved_connections +
NumWalSenders() - max_wal_senders) connection slots are available.

And the last question how to control the number of reserved slots for
replication. There are two options:
1. We can introduce a new GUC for that: replication_reserved_connections
2. Or we can just use the value of max_wal_senders

Personally, I more like the second option.

Attached patch implements above described functionality.
Feedback is very appretiated.

Regards,
--
Alexander Kukushkin