Improved control over frame size in zstd filter. (#2081)

Improved control over frame size in zstd filter. (#2081)

Instead of just `min-frame-size` and `max-frame-size`, we now have four
separate options:

* `min-frame-in` delays the creation of a new frame on flush until the
uncompressed size of the current frame passes a certain threshold.

* `min-frame-out` delays the creation of a new frame on flush until the
compressed size of the current frame passes a certain threshold.

* `max-frame-in` forces the creation of a new frame as soon as possible
after the uncompressed size of the current frame reaches a certain
limit.

* `max-frame-out` forces the creation of a new frame as soon as possible
after the compressed size of the current frame reaches a certain limit.

We now also support `k`, `kB`, `M`, `MB`, `G` and `GB` suffixes for all
four options.

The old options are retained as aliases for the corresponding new
options.