Patch: New GUC prepared_statement_limit to limit memory used by prepared statements

Hello,

attached you find a patch that adds a new GUC:

prepared_statement_limit:

        Specifies the maximum amount of memory used in each session to
cache
        parsed-and-rewritten queries and execution plans. This affects
the maximum memory
        a backend threads will reserve when many prepared statements
are used.
        The default value of 0 disables this setting, but it is
recommended to set this
        value to a bit lower than the maximum memory a backend worker
thread should reserve
        permanently.

If the GUC is configured after each save of a CachedPlanSource, or after
creating a CachedPlan from it, the function
EnforcePreparedStatementLimit is called now. It checks the mem usage of
the existing saved CachedPlanSources and invalidates the query_list and
the gplan if available until the memory limit is met again.

CachedPlanSource are removed-and-tailadded in the saved_plan_list
everytime GetCachedPlan is called on them so it can be used as a LRU list.

I also reworked ResetPlanCache, PlanCacheRelCallback and
PlanCacheObjectCallback a bit so when a CachedPlanSource is invalidated
the query_list is not only marked as invalid but it is also fully
released to free memory here.

Regards,
Daniel Migowski

PS(at)Konstantin: This patch also includes the CachedPlanMemoryUsage
function you like, maybe you like the review the patch for me?