dm-cache-policy-mq: fix alloc_entry to address callers (insert_in*cache()) requirement to return NULL
dm-cache-policy-mq: avoid code duplication in mq_{set,clear}_dirty()
dm-cache-policy-mq: add shim changes (bit field) and change to allow for the background shim to work
dm-cache-policy-background: convert to a shim policy stackable on top of eg. 'mq'
dm-cache-policy-hints: convert to a shim policy stackable on top of eg. 'mq'
dm-cache-policy-debug: convert to a shim policy stackable on top of eg. 'mq'
dm-cache-policy-basic: remove now superfluous next_ditty_block method in favour of the background policy shim
Makefile,Kconfig: add debug policy; adjust text for shim policies (background,debug,hints); move basic policy to DM_TEST_TARGETS