Port to version 4.1 of the KVS library: lots of option changes, we can
now specify KVS_O_CREATE in all cases, and we have a real KVS_O_TRUNCATE
flag.
Rename uri_source to be wt_source, I think it's a bit cleaner than
talking about URIs.
Upgrade to using KVS namespaces instead of imposing our own namespaces
on top of the underlying KVS store.
Merge pull request #575 from wiredtiger/memrata-txn

KVS library version 4.1 (upgrade to use the KVS name-spaces).