Various database migration fixes (#25209)

* String columns with a length higher then 4000 are converted into a CLOB columns automagically - we have to respect this when migrating

* Adding schema migration tests to prevent unnecessary and non-sense migration steps
Fix Oracle autoincrement and unsigned handling

* Fix sqlite integer type for autoincrement

* Use lower case table names - fixes pg

* Fix postgres with default -1 - this only affect pg 9.4 servers - 9.5 seems to work fine