cache: introduce cache_pool_max_chunks

cache: introduce cache_pool_max_chunks

Introduce 'hard limit' for max number of cache chunks.
When cache target operates with too many chunks (>10e6).

When user is aware of related possible troubles he
may increase the limit in lvm.conf.

Also verbosely inform user about possible solution.

Code works for both lvcreate and lvconvert.

Lvconvert fully supports change of chunk_size when caching LV
(and validates for compatible settings).