This bug is created as a clone of upstream ticket:
https://fedorahosted.org/389/ticket/48617

Follow on from #48384

Now that we have working ram and sanity checks, we need to make sure they work together properly.

Right now, as the server starts we are checking the backends in isolation.

Lets say we have 1GB of ram. We have two backends, that each request 768MB.

Both would pass the sanity check, as they check the free memory at the time, independent of each other. Then both start and over time would allocate up to 1.5GB potentially causing swap or worse OOM conditions.

Our backend sizing checks should check the sum of all backends and all caches together, before declaring "sane".

This will have to be done pre-backend init.

I propose:

* Remove the "fuzzy" second backend check that doesn't really work. It's not concrete.
* Move the db sizing checks out from ldbm plugins.
* Expose a plugin function to return the cache sizings and values. We should also have backends expose their dn and entry cache sizes.
* Alter the server start up procedure such that:

{{{
start slapd
load the plugins (but don't start the DB's / backend yet)
trigger each backend to "attempt" the autosizing procedure based on the admin's rules.
retrieve *all* the cache sizings
validate the sum of all caches
iff valid:
    start the backends and dbs
else:
    stop the server with lots of big warnings, and the complete set of information related to memory and cache sizings.
}}}