ATS runs without cache or partial cache on disk errors

Problem:
If ATS fails to initialize the cache (none of the disks were accessible), the behavior depends on proxy.config.http.wait_for_cache:
If wait_for_cache = 0, it will listen for requests and serve the requests (by fetching from origin/parent/peer).
If wait_for_cache = 1, it will never listen for requests. This is almost like a hang.

We would like to change this so that we can take some action when the cache fails to initialize (even partially):
Proposed Solution:
Define a new variable: proxy.config.http.cache.required
Value range: 0-2
0 (default) - Do nothing
1 - Abort trafficserver if it failed to initialize all the disks/volumes
2 - Abort trafficserver if it failed to initialize even one of the disks or volumes.

Preconditions for this new behavior are:
proxy.config.http.cache.required = 1 (HTTP caching enabled) and proxy.config.http.wait_for_cache = 1.