breaking change -- __crr_clock.site_id is now nullable and receives null entries

[tantaman]

The site_id column in clock tables is now nullable. This is to reduce data written for local writes where the site id is implicitly the current database.

If you have existing databases, either drop the %s__crr_clock tables and re-create them via crsql_as_crr(%s) or alter them to:

1. update %s__crr_clock SET site_id = NULL where site_id = 0
2. alter the site_id column to be nullable