Store previous month data in own partition

[astefanova]

Previous month data is no longer stored in current month partition(s). Instead we store the data in previous month partition.

As the previous month partition might be taken offline (or deleted) we safeguard with SLACK. Data for previous month is accepted only up to SLACK window expiration.

[cfdreddbot]

Hey astefanova!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/161245963

The labels on this github issue will be updated when the story is started.

[codecov[bot]]

Codecov Report

Merging #1078 into master will increase coverage by 0.01%. The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #1078      +/-   ##
==========================================
+ Coverage   95.22%   95.23%   +0.01%     
==========================================
  Files         125      125              
  Lines        3494     3506      +12     
==========================================
+ Hits         3327     3339      +12     
  Misses        167      167

Impacted Files	Coverage Δ
tools/mocha/src/lib/eventually.js	50% <0%> (ø)	:arrow_up:
...ggregation/aggregator/src/lib/aggregator-config.js	100% <100%> (ø)	:arrow_up:
...regation/accumulator/src/lib/accumulator-config.js	100% <100%> (ø)	:arrow_up:
lib/metering/meter/src/lib/message-handler.js	100% <100%> (ø)	:arrow_up:

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