Summary:
The default IO priority of WritableFiles is IO_TOTAL, meaning that
they will bypass the rate limiter if it's passed in the options.
This change allows to pass an io priority in construction, so that by
setting IO_LOW or IO_HIGH the rate limit will be honored.
It also fixes a minor bug: SstFileWriter's copy and move constructor
are not disabled and incorrect, as any copy/move will result in a
double free. Switching to unique_ptr makes the object correctly
movable and non-copyable as expected.
Also fix minor style inconsistencies.
Closes https://github.com/facebook/rocksdb/pull/2335
Differential Revision: D5113260
Pulled By: sagar0
fbshipit-source-id: e084236e7ff0b50a56cbeceaa9fedd5e210bf9f8
