* Track applied migrations to skip column checks on startup
Add a migrations table that records which migrations have been applied.
On boot, load the set of applied names in one query and only run new ones.
A fully migrated database now does 1 query instead of ~12 HasColumn/HasTable
checks.
Fresh databases created via CreateSchema record all migrations as already
applied. Old databases get the migrations table on first MigrateSchema call
and each migration is recorded after it runs.
Closes#54
* Add benchmark for MigrateSchema on fully migrated database
* Optimize MigrateSchema to single query for fully migrated databases
Skip HasTable/HasColumn checks when the migrations table already exists.
A fully migrated database now does one SELECT instead of ~12 individual
column and table checks.
* Add migration docs and link from architecture
* Add test for upgrade from fully migrated database without migrations table
