sql: add support for workflow retry params

[venkat1109]

• Fixes fix sql to support workflow retry #1339
• Also contains improvements to test coverage and related fixes

Note to Reviewer:
The 'executions' table at this point has close to 50 columns. We could consider storing the mutable state as a blob in MySQL - however the pros/cons of doing so are not very clear at this time. So, I have decided to tackle this problem independently after getting some data on the performance implications of such a change.

Pros/Cons to having too many columns are below. The I/O cost is likely to be the same with both blob and wide rows

• Cons

  • CPU cost on the MySQL server to do conversions to/from blobs obtained from InnoDB
  • Mutable state (executions) is something we expect to add/deprecate columns frequently. In a sharded mysql cluster, schema changes may be hard i.e. all storage shards must agree on the new schema despite operating independently - this may or may not add operational complexity

• Pros

  • Allows for joins on non-primary key fields - but we likely don't need this flexibility