Provide the ability to do a separate cache-hint for cloning.

Cloning is one of the very problematic places for high-latency IO
as it reads baskets one-at-a-time.  Hence, it makes a lot of sense
to use lazy-download for this case.  *However*, it may not be
desirable to enable lazy-download for all workflows!  Nor may it
be desirable for the workflow layers to enable lazy-download for
all merges (as some sites don't have the disk space).

This patch allows us to selectively turn on lazy-download only for
merge jobs at a given site by editing site-local-config.xml.

Per request from Dirk, this pull request is against 7_2_X.  It's fairly self-contained and I can back/forward port as necessary.

switch to ```std::unique_ptr<TTree>``` since it makes all management easier.