Support for READFORWARD in the caching tier

READFORWARD caching policy uses writeback for writes
and redirects reads to the back end starage tier.

Signed-off-by: Luis Pabón <lpabon@redhat.com>
Merge pull request #2089 from lpabon/lpabon_readforward

Support for READFORWARD in the caching tier

Reviewed-by: Sage Weil <sage@redhat.com>