[improve][client]pulsar-client-io thread has high CPU usage when client idle and batch message enable

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

Create a pulsar client and to initialize, send a little message to topic, then the pulsar-client-io thread(just 1 thread by default) cpu usage is 14%, but client is idle, no action for it.

the root cause is that batch message sending is enable, ProducerImpl would use pulsar-client-io thread to send message per 1ms by default

pulsar-client use EventLoopGroup as thread pool, the thread name is "pulsar-client-io"，EpollEventLoopGroup is preferred.
When switch to NioEventLoopGroup, the CPU usage just reduce little to 9.6%

### Solution

Use ScheduledExecutorService to send batch message to replace the EventLoopGroup. It would reduce CPU usage less than 1%

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Start by tracing batch-message scheduling in ProducerImpl and how the pulsar-client-io EventLoopGroup currently triggers sends. Compare that path with the proposed ScheduledExecutorService approach; done means idle clients keep batching behavior while reducing pulsar-client-io CPU usage to below 1%.

Written by the indexing model from the issue text.