ATM, the replication logic sends all dry-run requests in parallel,
which might overwhelm the ZFS pool on the sending side.
Since we use rpc/dataconn for dry sends, this also opens one TCP
connection per dry-run request.
Use a sempahore to limit the degree of concurrency where we know it is a
problem ATM.
As indicated by the comments, the cleaner solution would involve some
kind of 'resource exhaustion' error code.
refs #161
refs #164
