design doc with proposed changes for message and state tracking

[jipperinbham]

the plan is to leave this PR open while the implementation is occurring so we can update the doc with any specifics or changes along the way.

fixes #33

• remove any existing code
• reader state
• commit log
• writer restart
• reader resume
• log truncation
• error handling

[codecov]

Codecov Report

Merging #323 into master will increase coverage by 1.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   84.44%   86.43%   +1.98%     
==========================================
  Files          23       59      +36     
  Lines        1183     3457    +2274     
==========================================
+ Hits          999     2988    +1989     
- Misses        123      310     +187     
- Partials       61      159      +98

Impacted Files	Coverage Δ
adaptor/elasticsearch/transport.go	100% <0%> (ø)
adaptor/rabbitmq/writer.go	100% <0%> (ø)
adaptor/elasticsearch/clients/v2/writer.go	86.76% <0%> (ø)
function/rename/rename.go	100% <0%> (ø)
adaptor/rethinkdb/writer.go	86.36% <0%> (ø)
adaptor/elasticsearch/elasticsearch.go	92.1% <0%> (ø)
message/message.go	100% <0%> (ø)
pipe/pipe.go	100% <0%> (ø)
adaptor/mongodb/client.go	100% <0%> (ø)
function/omit/omitter.go	100% <0%> (ø)
... and 27 more

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