Optimization opportunities found while converting TableInfo

[tantaman]

• [ ] move causal length tracking to the primary key lookaside table. This removes a self-join on the clock table
• [ ] remove ensureTableInfosAreUpToDate -- we can proactively keep these up to date from the create_crr methods, removing an entire query from every merge operation.
• [x] remove crsql_get_table
• [x] prema prepare CLOCK_TABLES_SELECT
• [x] remove BTreeMap
• [ ] remove recreate_db_version_stmt
• [x] next_db_version / fill_db_Version_if_needed - we can check the pragma once per transaction. Need to save some bit that states we checked the pragma already and reset on tx commit or rollback.

[tantaman]

Found possible bugs:

• PK update should also update clocks for rows to move them to the new PK. They're not dropped but moved! Junction table test should now work?

This is being fixed in #344