Fixed receive buffers shared between SeedLink connections.

This should fix obspy/obspy#561.
Fixed spuriously declared class variables.

The way the trace variable was declared allowed for setting a
class variable, effectively rendering every getTrace() call on
every packet useless.

The blockette class variable was unused entirely.
Fixed spuriously declared class variables.

The way the selectors variable was declared led to the following bug:

>>> station1 = SLNetStation('', '', None, -1, None)
>>> station2 = SLNetStation('', '', None, -1, None)
>>> station1.appendSelectors('FOO')
1
>>> station2.getSelectors()
['FOO']
Changelog for obspy.seedlink bugfixes
Added issue number to test case.
Merge pull request #769 from bmorg/fix_seedlink_multithread

Fixed obspy.seedlink bugs caused by mixed usage of class/instance attributes. Fixes #561.
correct some minor flake8 accidentally introduced by #769