vinyl: split vy_range_compact into begin and commit stages
vinyl: split vy_range_compact into begin and commit stages

Call 'begin' from task->execute and 'commit' from task->complete.
'begin' writes runs to the disk, while 'commit' manipulates the range
tree, renames range files and updates range index. This is a step
towards removal of index->lock.

mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool