[Bug] Fixes in MsalSessionTokenCacheProvider.cs

[jmprieur]

Which Version of Microsoft Identity Web are you using ?
Microsoft Identity Web 0.1.5-preview

** Things to fix:

• The lock object does not need to be static: MsalSessionTokenCacheProvider.cs#L125
• The session should not be committed (for instance MsalSessionTokenCacheProvider.cs#L96, but there are other lines). Commit is automatically called at the end of the request. If we do it prematurely, nobody can modify it any longer.

[jennyf19]

Included in 0.2.0-preview release