Make logger cachedproperty

[osherdp]

It fixes an error that occures when having a flow with too many blocks (more than max num of file descriptors)

[coveralls]

Pull Request Test Coverage Report for Build 1239

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 96 unchanged lines in 16 files lost coverage.
• Overall coverage increased (+0.6%) to 84.948%

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Files with Coverage Reduction	New Missed Lines	%
.tox/py35/lib/python3.5/site-packages/rotest/core/abstract_test.py	2	91.11%
.tox/py36/lib/python3.6/site-packages/rotest/core/abstract_test.py	2	91.11%
.tox/py27/lib/python2.7/site-packages/rotest/core/abstract_test.py	2	91.67%
.tox/py37/lib/python3.7/site-packages/rotest/core/abstract_test.py	2	91.11%
.tox/py27/lib/python2.7/site-packages/rotest/core/runners/multiprocess/manager/message_handler.py	3	83.54%
.tox/py35/lib/python3.5/site-packages/rotest/core/runners/multiprocess/manager/message_handler.py	3	83.54%
.tox/py37/lib/python3.7/site-packages/rotest/core/runners/multiprocess/manager/message_handler.py	3	83.54%
.tox/py36/lib/python3.6/site-packages/rotest/core/runners/multiprocess/manager/message_handler.py	3	83.54%
.tox/py27/lib/python2.7/site-packages/rotest/core/flow_component.py	5	91.36%
.tox/py36/lib/python3.6/site-packages/rotest/core/flow_component.py	5	91.36%

Totals
Change from base Build 1232:	0.6%
Covered Lines:	12292
Relevant Lines:	14470

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💛 - Coveralls