Support parallel execution of repoctl

[cassava]

Currently, repoctl does not take parallel execution into account. This can lead to cases where a repoctl add or repoctl update does bad stuff.

This is probably strongly related to issues #47, #49, #57, and #58.

[AladW]

I don't know the precise way in which repoctl writes to the local repository, but it should suffice to flock on the repository root. e.g.

exec 200< /home/custompkgs
( flock -w $wait_for_seconds -n 200 || exit 1
  # ... do stuff with repository
)