mongoBackend "redirect" to context provideres for queryContext and updateContext operations

This PR comprises the part of issue #499 regarding mongoBackend.
I will do my best trying to apply style to code additions. Don't hesitate to tell me again if you see I'm failing on it ;)

Only for code additions in functional PRs that only merge on "develop" branch. For style fixes in old part, you should use a hardening branch, to be merge first in "release/iotplatform-v1", then in "develop" (as usual with hardening work). Note that the style fix in old part should keep aligned in both "release/iotplatform-v1" and "develop" to avoid "comparison noise" between both branches.