Improve support for connID.

driver.listen now returns a connID. Error is passed as argument.
enumconverter is now case-sensitive.
StringRef'ize EnumConverter.
Added ofp.list_connections support.

Implement rpcListConns.
Implement part of rpcSetTimer.
Remove datapathMap from ApiServer (use Engine’s).
Fix exception thrown in remote_endpoint().
Improve “stop server” logging.