Factor out ICheckSum and replace with System.Security.Cryptography.HashAlgorithm

IChecksum was brought over from Java in order to plug in the CRC32 class.

However, .NET already has its own abstract class, HashAlgorithm that is used to as a base class for all cryptographic algorithms. There are also several 3rd party implementations of CRC32 that implement HashAlgorithm that we could use (provided we drop support for .NET framework < 4.6.1). Crc32.NET claims to be one of the fastest implementations.

IChecksum is only utilized in a couple of places, but Lucene.NET's types should also be renamed accordingly to show they are using HashAlgorigthm instead of IChecksum:

• BufferedChecksum > BufferedHashAlgorithm
• BufferedChecksumIndexInput > BufferedHashAlgorithmIndexInput

The APIs between IChecksum and HashAlgorithm differ, but being that they serve the exact same purpose we should be able to make this work with a bit of refactoring.