Add SENTINEL INFO-CACHE [masters...]

Sentinel queries the INFO from every master and from every replica of
every master.

We can cache the INFO results in Sentinel so Sentinel can be a single
place to quickly get all INFO output for an entire Sentinel monitoring
group.

This commit gives us SENTINEL INFO-CACHE in two forms:
  - SENTINEL INFO-CACHE — returns all masters and all replicas
  - SENTINEL INFO-CACHE master0 master1 ... masterN — vararg specify masters

Results are returned as a multibulk reply with two top-level entries
for each master.  The first entry for each master is the name of the master.
The second entry is a nested multibulk reply with the contents of INFO,
first for the master, then an additional entry for each of the
replicas.
Merge pull request #2148 from mattsta/feature/sentinel/info-cache

Add SENTINEL INFO-CACHE [masters...]