Fix two typos in bit operations.

- Define separate times for durations vs. absolute times.
- Make more consistent with existing precedents in KJ style.

fix some error messages

Apparently writev() imposes a limit on the vector size.  Work around that.  Not sure why libc doesn't already do this.

Make it possible for a message builder to incorporate a byte array not owned by it without copying.  Useful for injecting large data blobs into a message without copying their contents.  Access this functionality through the Orphanage interface.

Extend FlatArrayMessageReader to indicate where the message actually ended, in case it was not the end of the array.  Useful when reading an mmap'd file with a Cap'n Proto prefix.  Also fix a bug in Maybe dealing with DisallowConstCopy subclasses.

Cleaner disconnect handling.  Better fix for issue #71, and also simplifies the interface and improves robustness.

Make TwoPartyVatNetwork::onDrained() work correctly on the server side.  Previously only worked on the client side, where it's actually not very useful.  Fixes #71.  There's a deeper problem, though, that simply holding a capability received from the client will hold the dead connection open.

Allow DynamicEnum fields to be set with int/uint

Fix bug where receiving EOF on the RPC connection could cause RpcConnectionState to go into an infinite promise loop.  Strange that this did not cause trouble earlier.  Also some minor tweaks.

Fixes #67.  Also, prevent people from accidentally making MessageBuilder subclasses copyable/movable as brought up in issue #64.