Redo SQL migrations

[insipx]

Before 1.0 release and after the tracking issue is finished, all the migrations in migrations/ should be overhauled to clean it up. SQLX has changed since that folder was first created, and the migrations binary now supports up/down scripts as well.

These are all breaking (but pretty easy) changes which is why getting it done on 1.0 release would be best.

General Improvements

• rename block_num to just number wherever possible. This makes queries easier to read & less redundant.

Metadata Table

• Add UNIQUE constraint to version of metadata table Add UNIQUE constraint to version of metadata table #353
• rename metato metadata in metadata table
• consider adding more version information for metadata (apart from spec_version)
• Add id field to make LAG queries easier.