hostorder and failover_timeout for libpq

Hello hackers,

Couple of years ago Victor Wagner presented a patch [1] that introduced 
multiple hosts capability and also hostorder and failover_timeout 
parameters for libpq. Subsequently multi-host feature was reimplemented 
by Robert Haas and committed. Later target_session_attrs parameter was 
also added. In this thread I want to revisit hostorder and 
failover_timeout proposal.

'hostorder' defines the order in which postgres instances listed in 
connection string will be tried. Possible values are:
* sequential (default)
* random

Random order can be used, for instance, for maintaining load balancing 
(which is particularly useful in multi-master cluster, but also can be 
used to load-balance read-only connections to standbys).

'failover_timeout' specifies time span (in seconds) during which libpq 
would continue attempts to connect to the hosts listed in connection 
string. If failover_timeout is specified then libpq will loop over hosts 
again and again until either it successfully connects to one of the 
hosts or it runs out of time.

I reimplemented 'hostorder' and 'failover_timeout' parameters in the 
attached patch. I also took some documentation pieces from Victor 
Wagner's original patch. I'll be glad to see any comments and 
suggestions. Thanks!

[1] 
https://www.postgresql.org/message-id/flat/20150818041850.GA5092%40wagner.pp.ru

-- 
Ildar Musin
i.musin@postgrespro.ru