IF NOT EXISTS option for CREATE SERVER and CREATE USER MAPPING statements

I implemented IF NOT EXISTS option for CREATE SERVER and CREATE USER
MAPPING statements
for one of our customers.
I think other users can also find it useful for scripting and automated
tasks.
The patches themselves are small and simple. Documentation is updated as
well.

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Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company