Draft of metadata API with timestamp tracking capabilities.

[splatch]

Introduction of tag metadata which serves similar purpose to result set metadata for JDBC, but with constraints on key syntax. Softly related to #1471 and other drivers which are capable of providing timestamp information as a part of message exchanges.

In order to force registration of metadata keys, so they can be identified by callers, if needed, keys have non-string form.

Feature been discussed recently in this thread: https://lists.apache.org/thread/chc1p0czm7q33n1r57zbw7h11c7wzp4l but origins of feature come from older thread: https://lists.apache.org/thread/r3g4y6p7kst7z5bdpmccd8538h3hl80f

[chrisdutz]

I'm on holidays this week can have a look next Friday.

[chrisdutz]

It does feel a bit different from the other parts of our API ... but I guess I would simply need to play around with it a bit. I gess we can always change things in the future, if we want to give all parts the same feel.

[splatch]

@chrisdutz FYI I've separated API and SPI parts, now it is a bit closer to overall paradigm we have. Still metadata is immutable and a bit constrained in manipulation to avoid 1) putting too much there 2) relying too much on it.