SSL: Add configuration option to prefer server cipher order

By default, OpenSSL (and SSL/TLS in general) lets the client cipher
order take priority.  This is OK for browsers where the ciphers were
tuned, but few PostgreSQL client libraries make the cipher order
configurable.  So it makes sense to have the cipher order in
postgresql.conf take priority over client defaults.

This patch adds the setting "ssl_prefer_server_ciphers" that can be
turned on so that server cipher order is preferred.  Per discussion,
this now defaults to on.

From: Marko Kreen <markokr@gmail.com>
Reviewed-by: Adrian Klaver <adrian.klaver@gmail.com>
Expose qurey ID in pg_stat_statements view.

The query ID is the internal hash identifier of the statement,
and was not available in pg_stat_statements view so far.

Daniel Farina, Sameer Thakur and Peter Geoghegan, reviewed by me.
SSL: Support ECDH key exchange

This sets up ECDH key exchange, when compiling against OpenSSL that
supports EC.  Then the ECDHE-RSA and ECDHE-ECDSA cipher suites can be
used for SSL connections.  The latter one means that EC keys are now
usable.

The reason for EC key exchange is that it's faster than DHE and it
allows to go to higher security levels where RSA will be horribly slow.

There is also new GUC option ssl_ecdh_curve that specifies the curve
name used for ECDH.  It defaults to "prime256v1", which is the most
common curve in use in HTTPS.

From: Marko Kreen <markokr@gmail.com>
Reviewed-by: Adrian Klaver <adrian.klaver@gmail.com>
Fix a couple of typos

Noted by Peter Geoghegan
Fix performance regression in dblink connection speed.

Previous commit e5de601267d98c5d60df6de8d436685c7105d149 modified dblink
to ensure client encoding matched the server. However the added
PQsetClientEncoding() call added significant overhead. Restore original
performance in the common case where client encoding already matches
server encoding by doing nothing in that case. Applies to all active
branches.

Issue reported and work sponsored by Zonar Systems.
merge master