Keep log files in sync between server and client

[DavidPL1]

When a UI is not started in standalone mode and not running on the same machine as the server, log files need to be synced in order to be displayed. Copying a file is less of a problem than how: the philosophy of this application is optimizing resource usage, in this case keeping network traffic at a minimum, that's what this issue is about.

Possible optimizations:

• Only sync logs that are being watched in the UI
• ~Use rsync because incremental updates use less bandwidth~
• Send new lines via the opened socket

Proposed approach:

1. Client sends log request to server on 'show log'
2. Server registers all requested log files and requesting hosts (only one entry per host is needed, even if it is running multiple UIs, because they will look at the same directory)
3. In a separate thread server monitors for file changes of registered logs in a certain interval
4. ~Trigger rsync~ Send LogChangedEvent on server to all registered clients on file changes

This could also be applied to slave managers in #24 and the server could even forward log requests to the slave managers to directly connect the remote machine running the slave instance to machines running a UI instead of gathering remote logs and forwarding them to subscribers (major reduction of traffic on the server)

Maybe also interesting for #18

[DavidPL1]

Scratch all that. The python logging module supports SocketHandler to send logs over socket and MemoryHandler to buffer logs until an output of a certain level is emitted or the buffer is full (the buffered log is then flushed to another handler).

New approach: Collect all logs on the server by running SocketHandlers in combination with MemoryHandlers on remote hosts. On the UI machine use sshfs to create a symlink of the servers log location to access remote files with local file descriptors (super network efficient: sshfs with tail on a changing file only causes traffic when the file is changed)