Update ioq to 2.1.2

[iilyak]

Overview

Update ioq to 2.1.2 to fix flaky test when on powerful machines the latency in ioq is so small that we call log10 with 0
This update also drags additional changes. The full list of commits is:

• 5ff5921 Merge pull request fixes on _rewrite handling #15 from apache/ioq_update_hist
• b1ce76d Log from 0 is undefined
• 8ada5fa Merge pull request Add empty line to test GitHub Pull Requests #13 from apache/allow-for-dynamic-ioq-classes
• c0ce268 Add dynamic IOQ class documentation
• d6e7a78 Allow dynamic classes in is_valid_class
• 08c6bbb Rearrange IOQ config declarations into header file

Testing recommendations

Unfortunately it doesn't seem to be possible to write a test case specifically for this problem. The change is very slow and we would have to rely on review and existent unit tests.

Related Issues or Pull Requests

• Log from 0 is undefined couchdb-ioq#15

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation