Encompassing one source pollsters with common context

After per-source separation of static resources & discovery samples are
publishing with context of separate pollster, so if it is used arithmetic
transformer which handle different meters, it will fail.

Now samples are publishing from common source context, after all
source meters have been collected.

Change-Id: I0d186247b4b87fd94317318c180bdfb9c6da70fc
Closes-bug: #1394228
remove useless looping in pipeline

previously, the pipeline filtered at the publisher stage. this
logic has been moved to an earlier stage yet we still do processing
at publisher.

this patch removes legacy code. at worse, it will have no effect on
performance. at best, we publish quicker as we don't loop and
groupby needlessly for each batch of samples.

Change-Id: I2171c067bd8bb293a8478756f8b4938f312614a1
Closes-Bug: #1401176
Merge "Encompassing one source pollsters with common context"
Merge "remove useless looping in pipeline"