GH-264: Add support for upsert/delete via periodic merge flushes

[C0urante]

Addresses #264

Implements upsert/delete functionality as proposed in the collaborative design doc, fixes some bugs in the connector, and adds an embedded integration test framework and tests for upsert/delete.

Some notes:

• The README is updated to include instructions on running the new embedded integration tests. It should be fairly straightforward to run these if you're used to running the existing Docker-based integration tests as the same environment variables are used and, beyond that, all that's required is a Gradle one-liner.
• The integration testing framework added here can be expanded to include new tests and possibly address Replace Docker-based integration tests with embedded integration tests #285
• The SchemaRetriever API has been updated to differentiate between key and value schemas when invoking setLastSeen; this is done in a backwards-compatible fashion, so existing implementations of that interface should work with the updated API seamlessly.
• The MemorySchemaRetriever is updated to use that new API correctly (the existing one didn't differentiate between key and value schemas, which led to some buggy behavior that was unearthed during integration testing)
• The SchemaManager class is optimized to avoid redundant operations (which is useful in general but also specifically with upsert/delete enabled as create/update requests are much more likely in that case and the likelihood of them being redundant is fairly high)
• BigQuerySinkTask is tweaked to avoid creating multiple BigQuery and SchemaManager instances; both classes are thread-safe, so this should be fine
• Existing unit tests are modified to reflect some of these tweaks
• New unit tests are added for other changes

[C0urante]

cc @whynick1 @mtagle @criccomini

This is a big one. No rush, but it's up now if you want to take a look.

[codecov-commenter]

Codecov Report

Merging #286 into master will decrease coverage by 1.18%.
The diff coverage is 65.86%.

@@             Coverage Diff              @@
##             master     #286      +/-   ##
============================================
- Coverage     70.87%   69.68%   -1.19%     
- Complexity      301      379      +78     
============================================
  Files            32       37       +5     
  Lines          1538     2052     +514     
  Branches        164      203      +39     
============================================
+ Hits           1090     1430     +340     
- Misses          390      549     +159     
- Partials         58       73      +15     

Impacted Files	Coverage Δ	Complexity Δ
...nect/bigquery/convert/BigQuerySchemaConverter.java	97.56% <ø> (ø)	26.00 <0.00> (ø)
...bigquery/exception/ExpectedInterruptException.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...ect/bigquery/write/row/AdaptiveBigQueryWriter.java	40.90% <0.00%> (+0.28%)	6.00 <0.00> (ø)
...bigquery/write/row/UpsertDeleteBigQueryWriter.java	26.31% <26.31%> (ø)	1.00 <1.00> (?)
...om/wepay/kafka/connect/bigquery/SchemaManager.java	35.03% <28.33%> (-41.44%)	11.00 <10.00> (+2.00)	⬇️
...ka/connect/bigquery/config/BigQuerySinkConfig.java	75.30% <50.00%> (-6.47%)	29.00 <2.00> (ø)
...wepay/kafka/connect/bigquery/BigQuerySinkTask.java	63.57% <64.06%> (+4.62%)	47.00 <23.00> (+20.00)
...afka/connect/bigquery/write/batch/TableWriter.java	66.66% <72.72%> (ø)	5.00 <1.00> (ø)
...y/kafka/connect/bigquery/utils/TableNameUtils.java	75.00% <75.00%> (ø)	3.00 <3.00> (?)
...fka/connect/bigquery/write/batch/MergeBatches.java	82.53% <82.53%> (ø)	20.00 <20.00> (?)
... and 14 more