local

The files in the external resources file store cache are now

automatically cleaned up based on a least recently used strategy. The

structure in the cache is as follows:

- modules-2

- resources-2.1

- 0

- hash 1 (*)

- hash 2 (*)

- ...

- ...

The cache now keeps track of files accessed inside the marked hash

directories. The cleanup action then deletes all such files that have

not been accessed in the last 30 days.

- Moved the file store to live under `caches/modules-2` as the file store is protected by the lock on this directory. The external resource metadata is cached under this directory as well.

- Use a hash of the _content_ of the external resource to calculate the resource's location in the file store, so that entries never need to be overwritten. Previously a hash of the _location_ of the external resource was used. Also include the base name from the URL to allow a human to get some idea of what each file contains.

- Use custom serialization rather than Java serialization for the external resource metadata cache.

- Renamed a couple of metadata caches to better reflect their purpose.

+review REVIEW-6562

- ExternalResourceAccessor takes care of downloading and caching URI resources

- Only cache HTTP and HTTPS URI resources

- Use a shared FileStore<String>

This reverts commit 3769869c339a2d4b753f92617641055adb3e4b16.