Add COPY FROM support

[ArjanSchouten]

Add copy in support for r2dbc postgresql driver. Postgresql supports COPY statements to insert data fast: https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-COPY

The implementation is inspired by CopyManager and QueryExecutor from https://github.com/pgjdbc/pgjdbc

Issue description

This PR contains the code for issue #183.

New Public APIs

    /**
     * Copy bulk data from client into a PostgreSQL table very fast.
     *
     * @param sql the COPY sql statement
     * @param stdin the ByteBuffer publisher
     * @return a {@link Mono} with the amount of rows inserted
     */
    Mono<Long> copyIn(String sql, Publisher<ByteBuffer> stdin);

Additional context

Note: we added a benchmark test. It turned out that reading the csv files causes much of the differences in our testing.

[mp911de]

Thanks a lot for your contribution. The API looks pretty decent. I added a few comments. Care to have a look?

[mp911de]

What's the reason for filtering IllegalArgumentException? Generally, when a failure happens, then the upstream subscription is canceled and the copy state remains partially active without closing it.

[ArjanSchouten]

Removed, it was meant for the IllegalArgumentException thrown in startCopy but I moved the doOnError onto the backendMessages only. Thanks

[mp911de]

For the client exchange, it makes sense to start with a many-sink (Sinks.Many.asFlux()) where we keep a single conversation/publisher with the server open. Otherwise, we call exchange multiple times and that generates a bit of overhead and may interfere with other connection activity causing protocol resynchronization.

I can apply this change during the merge as it bears some complexity.

[ArjanSchouten]

I did an attempt but it is a little bit complex since we have to wait for the CopyInResponse before sending the data frames.

[harmvanderwal]

Are there any updates on when this'll be merged and released?

[mp911de]

Not yet, but I'd like to include this API for the 1.0 release. @harmvanderwal did you get a chance to build the PR locally so you can verify the API is suitable for your use case?

[harmvanderwal]

I did and it works perfect for my specific use case.

[mp911de]

Thank you for your contribution. That's merged and polished now.