TopologyCache is a collection of deserialized structures with
information about cluster topology. This currently includes data from
_pico_instance, _pico_replicaset, _pico_tier & _pico_service_route_table.
The info is automatically kept up to date with the corresponding system
tables. The TopologyCache also caches the immutable info related to the
current instance, like instance name, replicaset uuid, etc.

From now on we should be reading this data from TopologyCache whenever
possible instead of going directly to system tables as we were doing
previosly.

At the moment only the plugin RPC module has transitioned to using
TopologyCache but other modules should be refactored as well.
Especially governor.

Before this fix if requesting RPC by an invalid bucket_id we would send
an RPC to every replicaset to check if they have such a bucket_id.
This is not needed because we know the allowed range.

Now panics from plugin code are handled the same way panics from
picodata code, that is they log the backtrace to the tarantool logger
and then abort the process.

Introduce new parameters for alter system:
  - vdbe_max_steps
  - vtable_max_rows

Closes #1130

Add new files:
- storage/snapshot.rs: stuff related to raft snapshot
- storage/schema.rs: stuff related to DDL logic

Code is only moved, no changes to logic

This patch includes the following improvements:
- Use clock_lowres_signal_reset() instead of raw alarm().
- Use path_unlock() instead of raw fcntl().
- Use std's Command api instead of raw execvp().
- reattach_stderr() is no longer needed as of tarantool-sys (28cb2cfa127).