Fetch ddocs in the separate processes under their own supervisor
Read LRU options from the couch config

LRU options now read from the couch config, like the rest of the world
and not from ddoc_cache app's env.

The opts could be specified in default.oni as the following:

    [ddoc_cache]
    max_objects = 50000
    max_size = 104857600
    max_lifetime = 60000

The defaults are 100 Mb for the max size of an object in the cache,
one minute for an object's expiration time and no limit on a number
of the objects in the cache.
Change one_for_one supervisor to a custom gen_server process

Benchmark shows that a supervisor module is not performant enough
to keep up with the spawning frequency demand in ddoc_cache.

`ddoc_cache_fetcher_sup` was swaped for a `gen_server` based
custom supervisor with `ets` as a process regestry

BugzID: 28132