Merge branch 'fix-plugin-exception' of github.com:Jim8y/neo into fix-…

…plugin-exception

* 'fix-plugin-exception' of github.com:Jim8y/neo:
  Update src/Neo/Plugins/UnhandledExceptionPolicy.cs
  ensure leveldb is not used in multithread env
  Revert "Revert "Plugin unhandled exception (neo-project#3349)" (neo-project#3366)"

# Conflicts:
#	src/Neo/Ledger/Blockchain.cs
#	src/Neo/Plugins/UnhandledExceptionPolicy.cs
#	tests/Neo.UnitTests/Plugins/TestPlugin.cs
#	tests/Neo.UnitTests/Plugins/UT_Plugin.cs