Add support for COPY IN protocol

[flimzy]

This PR is still a bit rough. It does not have tests for corner cases (most important: when client sends CopyFail), and I'm not entirely sure that the structure is as simple as it can be.

But I'd love to get some feedback on the approach.

Two main API changes:

1. CopyIn added to the DataWriter type.
2. The Execute method in the PortalCache interface needs an additional argument. I've implemented this in a backward-compatible way, though.

Still to do:

• Clean up commit history
• What error code should the server return when the client sends a CopyFail?
• Test for CopyFail from client
• Test for Text format
• Test for mixed column formats
• Should we provide any parsing of the copy stream? IMO, probably not by default (I don't need to parse it for my own needs), but maybe as helper utilities, in future PRs, if needed by someone?

Note that this does not implement CopyOutResponse or CopyBothResponse. I don't (yet) need either of these. But with the groundwork in this PR, they should both be easier to accomplish if/when needed.

[jeroenrinzema]

Thanks for opening a PR! Left a few comments. Would like to explore if we could expose the copy implementation as a io interface.

[jeroenrinzema]

Much thanks @flimzy! I have implemented the last changes as discussed in the comments. This PR is ready to be merged!

[flimzy]

Thank you, @jeroenrinzema , for picking up the ball where I dropped it. And thanks for the great package!