When using rsync, wait for files to be available

Ref #507 (closed)

When using LIGHTMETER_LOGS_USE_RSYNC=true or -logs_use_rsync, in case the log files do not yet exist (not yet synchronized), controlcenter should not terminate with error; instead, it should wait until files are completely synchronized.

This MR introduces a filesystem watcher that tries to identify when a synchronization is finished (on rsync, it creates a file move followed by a chmod in the directory), and also requires that that all required files exist.