Allow disabling closing the sharedOperationExecutor

[alxhill]

Currently, if you pass in an executorService to the client, it closes it when the client itself is closed. This means users who do the following:

try (Client client = createClient(executorService)) {
   ///
}

will cause any other usage of the same executorService to fail. We want to use a single shared executor service for our whole server for all clients, so would prefer it doesn't shut down when this pattern is used.

[chernser]

Good day, @alxhill !
Accepting an executor was made it let users customize behavior but your use-case is valid - for the client it is a borrowed resource and we might not to shutdown it. I will think about the fix - I would like to avoid another flag in configuration for that but changing behavior is also not good.

Just a note about such usage (I think, you know this, but just to note):

• internal http client has connection pool so it might need to be disabled com.clickhouse.client.api.Client.Builder#enableConnectionPool
• using async operations inside the client doesn't make sense if client instance is now shared.
• application can put each query task in owned executor service in this case - anyway client is disposed after use.