Feature/in memory data context id counters

[ericburcham]

Several changes:

• Updated Identity strategies to be non-static.
• Added tests to assert that incrementable (short, int, long) identity strategies reset to the default of 0 when an InMemoryDataContext instance is overwritten with a new one.
• Updates to LastValue are (still) thread-safe.
• Gave some IdentityStrategy<TType, TIdentity> properties protected setters. Cleaned up affected tests. Commented all API public surface area in the IIdentityStragy interface and its implementations.
• All but integration tests are passing, and I cannot run them successfully without a database instance.