Version 2.0.0 release

This is the first version of mangos v2. This is built on top of mangos v1, but includes a number of breaking changes.

• The compat package is gone.

• A new OpenContext and support API for building concurrent applications is available for req, rep, surveyor, respondent, and sub protocols.

• Dialing is "synchronous" by default now (so if a peer is down at first, the caller will get a chance to notice.) . This is tunable by the OptionDialAsynch socket option for dialers.

• The old Port API has been completely revamped with a better Pipe API, which includes notifications before and after the pipe has been attached to the socket (and also detached from the socket).

• Significant performance improvements have been made.

• The REQ/REP pattern is much more responsive to the case where a peer drops while a request is outstanding.

• It is now sufficient to just import a transport packet, the old method of explicitly adding a transport is gone.

• The underlying API for transport and protocol implementations has undergone quite a lot of change. These APIs are still not really stable, so and may change further. (We consider these APIs "internal" for now.)