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).

This patch boils down to the following improvements:

* Various linkage-related shenanigans have been documented and revamped.
* All logic to build tarantool-sys is now encapsulated in tarantool-build.
* Most of the magic `println!`s are now encapsulated in build-rs-helpers.
* Unsound function `export_symbols` has been dropped in favor of
  explicit linker flags (`--require-defined` for linux & `-u` for mac).
* `TarantoolBuildRoot` is now in charge of building and linking
  tarantool libs. It defines the build tree via path helper functions.
* Picodata's `build.rs` should be a little easier to read.