Add message dump capability to stasis cache layer

The cache dump mechanism allows the developer to retreive multiple
items of a given type (or of all types) from the cache residing in a
stasis caching topic in addition to the existing single-item cache
retreival mechanism.  This also adds to the caching unit tests to
ensure that the new cache dump mechanism is functioning properly.

Review: https://reviewboard.asterisk.org/r/2367/
(issue ASTERISK-21097)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@382705 65c4cc65-6c06-0410-ace0-fbb531ad65f3