block manager: reduce file cache to 40% of total fds
block manager: reduce file cache to 40% of total fds

Some management applications (including Cloudera Manager) monitor the number
of open fds belonging to each process, and warn when that number exceeds 50%
of the process' configured limit. By claiming half of the limit, the block
manager's file cache is pretty much guaranteed to trigger this warning.

These monitoring thresholds are reconfigurable, but since our 50% limit was
chosen pretty much arbitrarily, we can also (arbitrarily) drop it to, say,
40% in order to avoid tripping the warning in the first place. The extra 10%
should be enough for the (low) number of fds used by open WAL segments.

Change-Id: I67427bd05245c98315dca4140629976f67e2c6ed
Reviewed-on: http://gerrit.cloudera.org:8080/6094
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@apache.org>
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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