Fix a problem where stale old, closed ETW sessions might be turned on.

Fix a problem where stale old, closed ETW sessions might be turned on.

The ES_SESSION_INFO define was created to be turned OFF as a last resort for the Nuget version of eventSource if you could not get
 access to a windows API for getting ETW sessions (UWP Apps were not allowed to get this API).  Unfortunately becasue this was OFF
by default when the code was moved to CoreCLR it naturally disabled ETW_SESSION_INFO.

The fix is to add  an addition || !ES_BUILD_STANDALONE condition which insures better code is used whenever we are NOT the nuget
pakcage (ES_BUILD_STANDALONE means we are compiling the Nuget package)   I have added some comments to the code that is used
for the NUGET case to indicate is current difficiencies.