vinyl: split vy_dump 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. After this change we
don't need index->lock anymore - the next patch gets rid of it.

This patch makes vy_task look like a real mess - we probably should keep
arguments for different tasks as structs in a union. Also, the wrappers
around vy_dump_{begin,commit} as well as vy_range_compact_{begin,commit}
doesn't look very helpful. But let's leave mere cleanups and refactoring
up to the following patches.

Closes #1685