Single Thread for KEX Handling Causes Increased SSH Latency with High Concurrent Connections

### Version

2.12.1

### Bug description

The issue is related to [Apache MINA SSHD #458](https://github.com/apache/mina-sshd/issues/458).

In version 2.12.1, a singleton thread pool was introduced for the Key Exchange (KEX) message handler flushing. This change has led to a significant increase in the time taken to establish SSH connections when a single client frequently connects to multiple servers. With only one thread available for KEX, this architecture becomes a bottleneck.

Our application frequently SSHs to over 10,000 servers to periodically poll specific information. Due to the single-threaded KEX, all 10,000 SSH connections now rely on the same thread, causing the average time to SSH into a server to increase. This directly impacts the application's polling frequency, leading to performance degradation.

### Actual behavior

A single thread is used for KEX, leading to increased SSH time when the client handles several thousand SSH connections concurrently.

### Expected behavior

Provide a configurable option to set the number of threads for KEX.

### Relevant log output

_No response_

### Other information

_No response_

Start by reviewing the KEX message-handler flushing change introduced in version 2.12.1 and the related Apache MINA SSHD issue #458. Trace how the shared KEX thread pool is created and configured, then identify concurrency or connection-establishment tests covering many simultaneous clients. Done means the KEX thread count can be configured and high-concurrency connection latency is no longer constrained to one thread.

Written by the indexing model from the issue text.