Improve startup time of Distributed engine.

[azat]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Improve startup time of Distributed engine.

Detailed description / Documentation draft:

• create directory monitors in parallel (this also includes rmdir in
  case of directory is empty, since even if the directory is empty it
  may take some time to remove it, due to waiting for journal or if the
  directory is large, i.e. it had lots of files before, since remember
  ext4 does not truncate the directory size on each unlink 1)
• initialize increment in parallel too (since it does readdir())