Support for r2dbc-spi's Optional Interface 'LifeCycle'

[jchrys]

While implementing r2dbc-spi, the interface 'LifeCycle' is optional and not mandated by the specification.
However, we believe it is important to provide support for this interface as it can benefit minority use cases.
You can refer to this link for more information: https://r2dbc.io/spec/1.0.0.RELEASE/spec/html/#lifecycle.

[svats0001]

Hi @jchrys,

Do these interfaces need to be implemented anywhere or do they just need to be created? I think MySqlConnection has to implement Lifecycle. I don't think Wrapped needs to be implemented anywhere, it's up to the end user. Closeable might have to be implemented by MySqlConnectionFactory etc.

[jchrys]

You're absolutely right. There is no need to implement the Wrapped interface for MySqlConnection and MySqlConnectionFactory, as they are not wrappers. If, however, we come across a class that wraps an r2dbc-spi implementation, it would be advisable to implement Wrapped in that case.

[mirromutth]

The MySqlConnection should to do nothing in postAllocate, only contains one job in preRelease, which is to rollback if it is in a transaction.

Which means postAllocate will return an empty Mono, and preRelease will be an alias as rollbackTransaction.

[mirromutth]

Hmmm, I noticed that the COM_RESET_CONNECTION command, which should be more suitable as preRelease, it resets almost all state, including temporary tables, transactions, etc.

[mirromutth]

Forget it, it turns out that COM_RESET_CONNECTION is not a good solution.

This command not being supported in 5.6.x and below, it will also cause all prepared statements to be cleared.

[jchrys]

#171 #175