Add optional RM reservation limit to memtrackers

If RM and per-query memory limits were enabled at the same time, the
per-query limit would be ignored if RM wanted to expand the memory
allocation. This change adds an optional reservation limit to a
memtracker. The original limit goes back to being a hard limit -
i.e. any attempt to consume more than that amount results in
failure. The RM reservation limit is the RM-allocated memory limit. If
that is exceeded it triggers the ExpandRmReservation() method, which tries
to retrieve more memory as long as the hard limit is observed.

The net effect is that per-query memory limits have the intended,
hard-limit effect, while the RM limits coexist nicely and can expand
with more memory as required.

At the same time, we change the precedence of various ways of suggesting
an initial reservation size so that the user can change the reservation
size via a query option (MEM_RESERVATION_SIZE).

Change-Id: I41bfa4eb1336810a8a5946f6be3472111a052144
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3134
Tested-by: jenkins
Reviewed-by: Henry Robinson <henry@cloudera.com>
(cherry picked from commit a422026b522af0d9c127848fdbd76117164a3a67)
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3393