Feature/fix ObjectDisposedException when execute same query on different db contexts

[IvoryRubble]

pull request to fix issue https://github.com/BlueshiftSoftware/EntityFrameworkCore/issues/41 changes:

• added test case for execute same async query twice on different dbContext instances
• removed compiling query from lambda expression in MongoDbDatabase.CompileQuery() method

[crhairr]

Thanks for the PR! I'll check this out and see if I can figure out what's wrong with the build.