Add pg_stash_advice contrib module.

Add pg_stash_advice contrib module.

This module allows plan advice strings to be provided automatically
from an in-memory advice stash. Advice stashes are stored in dynamic
shared memory and must be recreated and repopulated after a server
restart. If pg_stash_advice.stash_name is set to the name of an advice
stash, and if query identifiers are enabled, the query identifier
for each query will be looked up in the advice stash and the
associated advice string, if any, will be used each time that query
is planned.

Reviewed-by: Lukas Fittl <lukas@fittl.com>
Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Discussion: http://postgr.es/m/CA+TgmoaeNuHXQ60P3ZZqJLrSjP3L1KYokW9kPfGbWDyt+1t=Ng@mail.gmail.com