Configuration to disable expensive flushing in NLogTraceListener

Autoflush is enabled by default for Trace sources. Even if it is disabled, some busy Trace sources perform frequent flushing. This results in heavy load on targets. This patch allows users of NLogTraceListener to ignore flush requests from Trace sources. Application can perform flush on NLog level instead.

BTW, NLogTraceListener forwards NLog exceptions as harmless as network timeout to Trace sources, which is quite likely to crash the application. I didn't address this issue in this patch, because disabling the whole flush is IMO much better idea.