System resource exhaustion

[Danconnolly]

Its quite easy to exhaust all IP ports when using this from async code. It appears to create a new connection for each call. This manifests in errors such as JsonRpc(Transport(Minreq(IoError(Os { code: 99, kind: AddrNotAvailable, message: "Cannot assign requested address" })))).

See also issue #12.

Workaround is to add a delay into the calling code.