SSHFS watchdog doesn't notice thread exit

[Sploder12]

Describe the bug
The SSHFS server's watchdog doesn't have a way of verifying that the SFTP thread it watches is still alive. This causes the SSHFS server process to continue executing even if it is no longer functional. This causes mounted folders to hang the shell since the SSH session is still alive but SFTP is not.

To Reproduce

1. Create a SSHFS mount.
2. Cause SftpServer::run to exit by some means.
3. Observe the SSHFS server process running with no errors logged and not functional.

Expected behavior
The SSHFS watchdog notices the thread exit and takes appropriate actions so that there are logs and the shell doesn't hang.