Slave crashes when nuking release where files are in-progress

[hrxcodes]

Describe the bug
Nuking / Deleting / Moving / Wiping a release with active transfers might kill the slave process as some threads are writing/reading as the folder is being moved/removed on the backing device.

Autofreespace running before the slave has finished remerging would trigger this as well.

Expected behavior

• Abort+block transfers to paths being nuked or wiped, similar to glftpd's "this file is being deleted" and don't crash the slave

Maybe we can be a bit smarter on how/when we raise exceptions for missing files, most of the time the backing filesystem will not be the problem.

Additional context

Example of a crash:

drftpd/src/core/slave/src/main/java/org/drftpd/slave/vfs/RootCollection.java

Lines 264 to 272 in 5a5ac84

	public Root getRootForFile(String path) throws FileNotFoundException {
	for (Root root : _roots) {
	File file = new File(root.getPath() + PhysicalFile.separatorChar + path);
	if (file.exists()) {
	return root;
	}
	}
	throw new FileNotFoundException(path + " wasn't found in any root");
	}

DEBUG 18:41:07,397 [Slave Main Thread] org.drftpd.slave.Slave [] - Slave fetched org.drftpd.common.network.AsyncCommandArgument[index=f1,name=checksum,args=/linux/irunarch/btw.iso]
DEBUG 18:41:07,398 [AsyncCommandHandler - class org.drftpd.common.network.AsyncCommandArgument] org.drftpd.slave.Slave [] - Slave wrote response - AsyncResponseException exception is - /linux/irunarch/btw.iso wasn't found in any root
DEBUG 18:41:07,398 [AsyncCommandHandler - class org.drftpd.common.network.AsyncCommandArgument] org.drftpd.slave.Slave [] -
DEBUG 18:41:07,412 [Slave Main Thread] org.drftpd.slave.Slave [] - Lost connection to the master, may have been kicked offline
WARN  18:41:07,412 [Slave Main Thread] org.drftpd.slave.Slave [] - Shutdown() called
WARN  18:41:07,412 [Slave Main Thread] org.drftpd.slave.Slave [] - Closing _sin
WARN  18:41:07,412 [Slave Main Thread] org.drftpd.slave.Slave [] - Closing _sout
WARN  18:41:07,412 [Slave Main Thread] org.drftpd.slave.Slave [] - Closing _socket
INFO  18:41:07,412 [Slave Main Thread] org.drftpd.slave.Slave [] - Setting slave status online to: false