[IMPR] Implement EventStreams

eventstreams.py:

• EventStreams class
• setup the url from site.family and streamtype if it is not given
• setup timeout from config value
• set total number with set_maximum_items
• setup any filter with register_filter method
• streamfilter filters the events to be handled
• site_rc_listener method for pagegenerators.py with fallback to rcstream.py

family.py:

• rcstream_path and eventstream_path added

rcstream.py:

• use rcstream_path from family file instead of a default path
• print a deprecation warning when site_rc_listener is used

family.py:

• additional paths are provided for eventstreams and rcstreams

pagegenerators.py:

• use the new eventstream site_rc_listener

requirements.txt:

• bind sseclient instead of socketIO-client

pagegenerators_tests.py:

• test live recent changes with EventStreams

eventstreams_tests.py:

• test suite added

setup.py

• add core library dependency

Bug: T158943
Change-Id: I4e48784cc3a30c22cdb4882dbbebf0e5a68ff8c2