MB-7506: Allow percentages to change high and low watermarks

Can be set by specifying percentages of bucket's mem_quota

./cbepctl localhost:12000 set flush_param mem_high_wat 90%

Change-Id: I9873030901014f5f62a54db89e44e0875190a1e0
Reviewed-on: http://review.couchbase.org/25423
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>