Directory (file) cache never expires/shrinks

[Joshua Harlow]

In the file/dir backend there is a cache of all used files (saved in memory), this data is used to avoid re-reading the same flow/atom/task/logbook details/information again and again, but there is one larger problem, in that the information cached in that backend is never expired (or removed after a size limit is reached); this means for a long lived backend with many engines using it that the cache size can eventually get very big (instead of only reaching a given size and then evicting entries).

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/176104
Committed: https://git.openstack.org/cgit/openstack/taskflow/commit/?id=33e9ccc42566f4b666e04c1291bf01539288ffaf
Submitter: Jenkins
Branch: master

commit 33e9ccc42566f4b666e04c1291bf01539288ffaf
Author: Joshua Harlow <email address hidden>
Date: Tue Apr 21 12:49:06 2015 -0700

    Use a lru cache to limit the size of the internal file cache

    Instead of having an unbounded internal file cache which will
    eventually absorb all memory of the running/containing python
    process have the size of that cache be limited by an optionally
    provided size (and have eviction be based on how recent a cached
    entry was used).

    Fixes bug 1458248

    Change-Id: I5e59efc4edd51b05cfb1e67d3e7014e378e352aa