Fixed concurrency errors in MemoryCache when using dotnetcore

[sebslsch]

Fixed concurrency errors in MemoryCache (#101)

When using dotnetcore, adding a cache entry lead to InvalidOperationExceptions
if a new registration callback was added concurrently.

The issue was caused by re-using a single MemoryCacheEntryOptions for multiple
cache entries. This change resolves the issue by creating a new options
instance for each cache entry.

This causes a change of behavior with the eviction callbacks.
Previously, when using dotnetcore, all registered callbacks would be kept and
invoked on a cache eviction.
Now, only the last registered callback will be invoked. This behavior is
consistent with the implementation for .NET Framework 4.5.2.

The default implementation of CacheRepository will register a new callback in
the constructor of each instance.
Because each callback modifies a static class field, it should not matter
which callback is invoked.

It might, however, affect custom implementations.

[gautema]

Changes included in version 1..32.2