Introduce customization hook methods. Rename DefaultTypedGenericExecuteSpec to DefaultTypedExecuteSpec and GenericExecuteSpecSupport to ExecuteSpecSupport as types are not tied to generic execution. Encapsulate fields in DatabaseClient builders.
We now use java.lang.Long as fallback Id type for entities that do not declare an explicit Id property. Previously we used ObjectId which was a left-over from the initial draft of MappingRelationalEntityInformation.
Declare interface exposing inConnection(…) methods to encapsulate types implementing functionality that is applies within a connection scope. Move FetchSpec and SqlResult implementations to top-level types.
DatabaseClient now creates functions. The actual invocation/execution takes place from DefaultFetchFunctions (FetchSpec) to keep stateful resources in the scope of the execution. execute()/executeMany() use Flux.usingWhen/Mono.usingWhen to release connections after their usage.