Allow reading and writing messages from sockets in async mode

[LasseBlaauwbroek]

Resolves #298 (see also capnproto/capnproto#1542). Step towards #311.

This PR build on top of #310, only the last commit is relevant.

@kentonv I'd also like to implement read_async_packed() and write_async_packed() if possible. But I don't see any obvious functions in the C++ implementation that make this possible. And I can find surprisingly little information from other people that may have tried this before. The most relevant I can find is https://groups.google.com/g/capnproto/c/X344P1vATsU/m/5qxvGtDvAQAJ How would you suggest this could be implemented? Or is there currently no way? Thanks.

[kentonv]

When I read packed encoding in async mode in C++, I do it by reading the whole packed message into memory in advance, then parsing it (from memory) using the synchronous implementation. Similarly for writing, write to a buffer first.

[LasseBlaauwbroek]

Okay, so presumably you propose to populate a BufferedInputStream with a packed message, which is then consumed by PackedMessageReader? If so, it looks to me like the only way of achieving this in async mode is to have some kind of framing around messages? Otherwise one would not know how large the packed message is. And the message cannot be read in small pieces, because the interface to do this (like BufferedInputStream::tryGetBuffer) is not async. Am I missing something here?

If my analysis is right, isn't this a limitation of the API? Theoretically, async reading of packed messages seems possible (if, perhaps, somewhat slow). It's just the API that doesn't allow it?

[kentonv]

Yeah to be honest I've just never used packed encoding in an async stream with multiple messages. It does seem like the library is missing something there.

[haata]

@LasseBlaauwbroek please rebase, otherwise it looks good.

[LasseBlaauwbroek]

Bad git for not resolving these conflicts automatically. Rebased.