keep a pool of connections open when dialing for drpc. this
makes it so that long lived clients (like lib/uplink's Project)
don't continue to use a bad connection forever. it also allows
for concurrent rpcs.
Change-Id: If649b286050e4f09c413fadc3e1ce88f5fc6e600
It provides an abstraction around the rpc details so that one
can use dprc or gprc with the same code. It subsumes using the
protobuf package directly for client interfaces as well as
the pkg/transport package to perform dials.
Change-Id: I8f5688bd71be8b0c766f13029128a77e5d46320b
