Clarify the limits of PM_CONTEXT_LOCAL when using pmwebd
pmwebd: update docs for graphite capabilities
pmwebapi.3: continue elaborating on -h local: vs. -L
pmwebapi: adapt to some libmicrohttpd's multiple "completed" callbacks

In some circumstances, we could receive two callbacks, and would try
to delete the postprocessor/context objects each time.  Now, clear the
state variable after delete so we don't try it again.

Tested by preexisting 660 test case, failing with libmicrohttpd 0.9.34-4.fc22.x86_64
pmwebd man page: point out rate-conversion
pmmgr todo++
Revert "pmdagluster: thread-based timeout for too-long gluster query"

This reverts commit bdd8ca889edcf376670aba0b30779feb2c7d9203
(correcting attribution).
pmdagluster: thread-based timeout for too-long gluster query

glusterfs commands taking a while during dusruptive events is
definitely not "social".  To work around the issue, I updated
the gluster pmda to place the vol info and profile commands
into a separate thread, with a timeout.

I've also added a log record showing when the timeout triggers
[Mon Oct 20 09:52:21] pmdagluster(5264) Info: Gluster command (gluster --xml volume profile repl info) took too long to respond, triggering timeout

Now with the same scenario, the agent doesn't get terminated by
pmcd and you can see the self heal process kick into life quite
well.
pmmgr: avoid some granularity-edge races

Changes to pmmgr proper involve active avoidance of granular-mode
period boundaries.  pmloggers are instructed to shut down one second
before, and new pmloggers are precluded from launching within that
transitional second.  This seems to make the resulting archives'
timespans match the intuitive expectations.
pmwebd todo++
Merge branch 'fche-merge' into dev
man page: fix typos on dbpmda.1 that Lukas noticed