Remove now-redundant locking layer from Python PMAPI wrapper
Remove now-redundant locking layer from Python PMAPI wrapper

We currently have two levels of locking for Python clients,
due to the original python wrapper predating the threading
work done in libpcp.  This change removes all locking from
the pcp.py layer (both the BigPythonLock & lastUsedContext).

In doing this, several dormant bugs were exposed (and fixed
at the same time), some quite subtle - happily the code is
much simplified, so these stood out:
- pmLookupDesc (error code not checked within the loop)
- pmSetMode (incorrect status checking, spurious zeroing)
- pmRecordSetup (missing unlock on error path)
- pmRecordAddHost (missing unlock on error path x 2)

I *think* (unrelated) issues remain in the following calls:
  pmErrStr_r, pmUnitsStr_r, pmIDStr_r, pmInDomStr_r
(the calls do not match the C calling convention - can this
be right?  surely not?  not sure though, maybe some Python
magic happens under the covers here).

andyvand/cygpcpfans,wuliming/pcp,adfernandes/pcp,ryandoyle/pcp_original,edwardt/pcp,wuliming/pcp,ryandoyle/pcp_original,edwardt/pcp,tjanez/pcp,wuliming/pcp,wuliming/pcp,prasincs/pcp,adfernandes/pcp,mbaldessari/pcp,prasincs/pcp,adfernandes/pcp,tjanez/pcp,aeg-aeg/pcpfans,tjanez/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,tjanez/pcp,edwardt/pcp,tjanez/pcp,wuliming/pcp,mbaldessari/pcp,ryandoyle/pcp_original,prasincs/pcp,edwardt/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,aeg-aeg/pcpfans,andyvand/cygpcpfans,andyvand/cygpcpfans,mbaldessari/pcp,ryandoyle/pcp_original,adfernandes/pcp,adfernandes/pcp,tjanez/pcp,tjanez/pcp,andyvand/cygpcpfans,adfernandes/pcp,andyvand/cygpcpfans,prasincs/pcp,andyvand/cygpcpfans,wuliming/pcp,edwardt/pcp,andyvand/cygpcpfans,mbaldessari/pcp,aeg-aeg/pcpfans,ryandoyle/pcp_original,prasincs/pcp,ryandoyle/pcp_original,ryandoyle/pcp_original,prasincs/pcp,mbaldessari/pcp,edwardt/pcp,mbaldessari/pcp,tjanez/pcp,edwardt/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,prasincs/pcp,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,prasincs/pcp,aeg-aeg/pcpfans,ryandoyle/pcp_original