Filezcache - cache s3 and other files on disk

This is a file caching system optimized for use with cowmachine
but then also be used with other services.

It has some unique properties:

 * Lookups return file references (if possible) which can be used with sendfile
 * Cached files can be streamed to requestors while they are being filled
 * On startup the cache is repopulated with existing files
 * Lookups return data formats directly useable for cowmachine serving

The system uses the file system to store files and a disk log. The files are stored
in `priv/data/` and the disk log in `priv/journal`.

The disk log is used to rebuild the cache after a start. All files are checked against
the checksum from the disk log, non-matching files are deleted from the cache.


    $ erl -pa ebin
    Erlang R15B03 (erts- [source] [64-bit] [smp:4:4] [async-threads:0] [kernel-poll:false]
    Eshell V5.9.3.1  (abort with ^G)
    1> application:start(crypto).
    2> application:start(filezcache).
    3> filezcache:lookup(mykey).
    {error, enoent}
    4> filezcache:insert(mykey, <<"foobar">>).
    5> filezcache:lookup(mykey).
    6> filezcache:insert(mykey, <<>>).
    7> filezcache:delete(mykey).

Configuration keys

 * `max_bytes` Maximum size of the cache in bytes, defaults to 10GiB (10737418240 bytes)
 * `journal_dir` Directory for the disk log, defaults to `priv/journal`
 * `data_dir` Directory for the cached files, defaults to `priv/data`


There are some known issues that need to be resolved:

 * On startup delete files that are unknown to the disk log
 * Add timeouts to `filezcache_entry` states `wait_for_data` and `streaming`
 * Extra intelligence in filezcache_entry to prevent evicting active entries during garbage collection