change namespacing of concurrency error classes

This would allow to rescue from a generic
ConcurrencyBalancer::Error.
refactor to use try-count as optional keyword-arg.
set instance vars for sequential batch parse

These instance variables are required for correct
error handling when performing inside the sequential queue.
move redis-wrapper into reuseable module.
document sidekiq testing style in test-files.
refactor to remove worker init duplication.
Omit extraneous `ruby-' prefix.
document the concurrency balancer.
Merge pull request #792 from ontohub/remove_ruby_version_prefix

Omit extraneous `ruby-' prefix.
use savepoint for nested sql transaction

This may be able to fix the #576 issue.

We assume that the problem was related to the fact that
the failing cycle-checker would fail the transaction
(as true nested transactions are not supported by PostgreSQL)
and the next query (the logic_id check, or now the projects query)
is still executed as part of the transaction-block. However the
transaction does not exist anymore so the system fails.

However `requires_new: true` will add a savepoint. So
if the cycle-check (the content of the transaction-block)
fails, it will only rollback to the savepoint, but the
transaction will still exist.
Merge pull request #724 from ontohub/521-adding_counters

521 adding counters to the feature selectors of ontology search
Mark "File browser" tab if selected.
Merge pull request #750 from ontohub/685-solving_multiple_ontology_concurrency

685 solving multiple ontology concurrency
Force download file on OWL API HTTP accept headers

Temporarily fixes #783.

The OWL tools somehow make two different HTTP requests.
One type has the accept header
"application/rdf+xml, application/xml; q=0.5, text/xml; q=0.3, */*; q=0.2"
while the other type has the accept header
"text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2".
However, they both are used to get the exact same .owl file.

This commit is about to be reverted as soon as the OWL tools send
consistent request headers.
Select "Content" tab for entities views.
Merge pull request #784 from ontohub/783-cooperate_with_owl_api

783 cooperate with owl api
Merge branch 'staging' into 759-clean_develop_ontohub_org
use code styleguide, simplify
Merge pull request #788 from ontohub/759-clean_develop_ontohub_org

759 clean develop ontohub org
Merge pull request #717 from ontohub/640-project_management

640 project management