Set level of repetable logs to debug

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

We're buried in tons of non-meaningful logs without any option to set the verbosity. Non-meaningful logs are:

• information about usage of deprecated endpoints
• 'before_request'/'request' logs which are useful for debugging hanging or slow endpoints, but only for debugging
• threadName, moduleName, lineNo which doesn't carry any useful information that can't be read from the log itself

Missing request logs can be easily replaced with metrics introduced by #908

What is the new behaviour?

• introducing DEBUG level that can be turned on via enable_debug_log option in configuration
• setting verbosity level of logs mentioned above to DEBUG