fix: allow RollingFileAppender to always run - #280

 * When using assets/logback.xml, main URL to watch was not set
   because an input stream was passed to GenericConfigurator
   instead of a URL. Without a main URL, reconfigure-on-change
   was disabled. This patch gets the assets resource as a
   URL instead to allow the reconfigure-on-change to proceed.

 * Special props were missing from the reconfiguration because
   the context was reset beforehand, but the special props were
   not reinserted into the context. This patch performs the
   necessary setup.

InvocationGate was originally added to improve performance in
scenarios of high frequency logging, as the rollover-condition
check (file size for SizeBasedTriggeringPolicy) could be
relatively expensive especially for large files. The "gate"
effectively ignored some logging/rollover events that occurred
close together in time.

However, this feature causes data loss from the ignored logging
events and oversized active log files from missed rollover events.

The feature itself seems like a bandaid for a larger performance
problem that should've been solved in a different manner, but it
ultimately caused worse problems. Thus, I'm removing InvocationGate
and allowing RollingFileAppender and its triggers to run freely.

…xternalStorageState() returning null (#204)