Consider supporting using `R2dbcDatabase` as the underlying Exposed database to generate SQLs in `DatabaseClient`

[ShreckYe]

Extract common functions with the blocking Database.

[ShreckYe]

Not so urgent if it doesn't bring much benefits, for example performance-wise as should be benchmarked in #28.

[ShreckYe]

Also this will be rendered moot if #3 is supported.