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.

Maksim Kaitmazian authored 4 months ago and Dmitry Ivanov committed 4 months ago

With these changes, a user can specify new default values for vdbe_max_steps
vtable_max_rows options in connection string.

For example, the following connection string sets both options to 42:
postgres://postgres:Passw0rd@localhost:5432?options=vtable_max_rows%3D42,vdbe_max_steps%3D42

Add alter system cluster_wal_max_size and cluster_wal_max_count parameters
which control the auto compaction of _raft_log system space based on
the size in bytes and number of tuples respectively.