os/bluestore: Add do_write_v2_compressed()

os/bluestore: Add do_write_v2_compressed()

Modify do_write_v2() to branch into do_write_v2_compressed().
Segmented and regular cases are recognized and handled properly.
New do_write_v2_compressed() oversees compression / recompression.

Make one Estimator per Collection.
It makes possible for estimator to learn in collection specific compressibility.
In write_v2_compressed use compressor already selected in choose_write_options.
Make Collection create Estimator on first use.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>