[C#] Compaction via session, update EntryCount, remove ShallowCopy

[badrishc]

• Move Compact to ClientSession so that user does not need to implement Copy/InPlaceCopy functions. Example:

  using (var session = store.NewSession(...))
    session.Compact(address, true);

• Update EntryCount and DumpDistribution to reflect only hash entries >= Log.BeginAddress
• (Internal) Remove ShallowCopy since we now have serialize

Fix #358

[badrishc]

This PR also simplifies log compaction. Basically, it moves the Compact call to ClientSession. This way, we can use the existing IFunctions to perform the compaction. These functions would already handle concurrency for normal operations. What this also implies is that ICompactionFunctions can be simplified to only contain IsDeleted(...) and user does not need to provide anything by default.

Usage Example

using (var session = store.NewSession(...))
   session.Compact(address, true);

We can deprecate the Compact operation on LogAccessor (i.e., store.Log.Compact(...)) - when we remove it, we can also remove the Copy and InPlaceCopy methods from the ICompactionFunctions interface.