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.

Yeah, I thought about this.  Questions:
- "tree_" -- we don't own this pointer, CMSSW does.  unique_ptr wouldn't work out here.
- "file_" / "mytree_" -- we own this, but there's a specific ordering that destruction must be done.  How do I guarantee this with unique_ptr?