mmlid: Simple cassandra migration library

[dirkmc]

As part of the solution to #1623 we will need to integrate a cassandra migration into boost.

We need to change the MinerAddr column from t1234 to f1234, however there are no built-in string manipulation functions, and yugabyte does not support user defined functions. So we need a migration library that can run golang migration functions.

We use goose to do this for sqlite and postgres, but it turns out that:

• goose doesn't support cassandra
• to add cassandra support is not straight-forward, because
• goose uses the sql.DB construct
  • there are no maintained cassandra drivers that implement sql.DB
  • there is a sort of sql.DB implementation of a cassandra client but it doesn't support transactions, which are required by goose

Given all of the above, I think we should write a simple migration library ourselves for cassandra that

• uses hard-coded migration functions with a name and a migrate function
• only does up migrations
• does not have any CLI tools
• creates a table migration_version with a name columns to keep track of cassandra migrations
• when boost starts up it
  • checks migration_version against the array of migration functions
  • if there are any migration functions that have not been run it
    • runs the function
    • adds a row for each one to the migration_version table