vinyl: remove dependency of scheduler on environment

Instead of storing a pointer to vy_env in vy_scheduler, let's:

 - Add pointers to tx_manager::read_views and vy_env::run_env to
   vy_scheduler struct. They are needed to create a write iterator
   for a dump/compaction task.

 - Add a callback to struct vy_scheduler that is called upon dump
   completion to free memory. This allows us to eliminate accesses
   vy_env::quota and vy_env::allocator from vy_scheduler code.

 - Move the assert that assures that the scheduler isn't started during
   local recovery from vy_scheduler_f() to vy_env_quota_exceeded_cb()
   callback so that we don't need to access vy_env::status from the
   scheduler code. Note, after this change we have to set vy_env::status
   to VINYL_ONLINE before calling vy_quota_set_limit(), because the
   latter might schedule a dump.

 - Check if we have anything to dump from vy_begin_checkpoint() instead
   of vy_scheduler_begin_checkpoint().

This will allow us to isolate the scheduler code in a separate file.