R2dbc connection pool returns multiple pooled objects pointing to same database connection

[Azukovskij]

R2dbc returns multiple pooled objects pointing to same database connection in case downstream gets disposed, e.g.

• Downstream Mono converted from Flux or other Publisher (e.g. any RxJava type)
• Dispose called from different thread, e.g. Flux.onError, Mono.onError invoked from timeout operator on different scheduler
• Number of results is limited via Flux.take

Dispose event in downstream causes ref.release().subscribe(); to be invoked that releases pooled references (without calling preDestroy or any kind of logging event). As a result another pending invocation ConnectionPool#create receives same database connection, that has not yet been closed and has not finished processing in initial subscription.

This causes connection sharing issues on higher load that lead to lost cursor and data consistency issues (when transactions are leaked).

Please see #210 for unit test reproducing the issue and proposed solution - in case more full-proof solution is required connection dispose hook can be added on PooledConnection via Cleaner