Skip Akavache 6.0

[ghuntley]

• Adopt NetStandard
• Use System.Reactive (i.e. rx3.0 bits)
• Make Akavache fully buildable on Windows /cc @ghuntley
• Support Realm.io as storage provider [RFC] [WIP] Support Realm.io as storage provider #292
• Throw ObjectDisposedException and ArgumentNullException synchronously Throw ObjectDisposedException and ArgumentNullException synchronously #286
• Figure out what to do with ISecureBlobCache on Android and iOS, since the ISecureBlobCache isn't actually encrypted on these platforms EncryptionProvider is not protecting data under monodroid, monotouch, or monomac #190

Potentially also replace the current implementation of ConcurrentDictionary with the new one from Reference Source.

[patroza]

@ghuntley what about SQLite3-1.0.1, for it's awesome multi platform support?
I guess that falls under the 'Adopt NetStandard'