Spring Boot GA + JSON support

Changes:

[#24] Use Spring Boot 2.3's GA Version
[#25] Custom adapters converting between JOOQ and R2DBC types
[#27] Support org.jooq.JSON with r2dbc-mysql
[#26] Support org.jooq.JSON and org.jooq.JSONB with r2dbc-postgresql
[#10] Run unit tests on all supported databases automatically

Reactive JOOQ records

Changes:

[#2] Add support for UpdatableRecord.insert/update/delete
[#7] Make fetch() return table records if possible
[#4] Add support for UpdatableRecord.store
[#8] Update generated keys in TableRecord after ReactiveJooq.insert()
[#5] Add support for UpdatableRecord.refresh
[#17] Add executeReturning() and executeReturningOne() for insert/update
[#9] Make unit tests runnable with all database types
[#19] Fix MySQL support
[#21] Separate record logic and query logic