tools restore parallelism

[s-sabitova]

Currently tools restore is suitable for importing data within a single process. Sometimes it's is not fast enough, eg when several large tables need to be restored.

The idea is to allow several tools restore processes to run in parallel. Each process should restore a chunk of a table.

In order to support the idea, the following should be done:

• Allow tools restore to write into existing tables (currently it's restricted)
• When launching a restore process, user should be able to specify the list of chunks (files that were dumped) which will be restored by this process.