Rework FSInputChecker and FSOutputSummer to support checksum code sharing between ChecksumFileSystem and block level crc dfs

Comment from Doug in HADOOP-1134:
I'd prefer it if the CRC code could be shared with CheckSumFileSystem. In particular, it seems to me that FSInputChecker and FSOutputSummer could be extended to support pluggable sources and sinks for checksums, respectively, and DFSDataInputStream and DFSDataOutputStream could use these. Advantages of this are: (a) single implementation of checksum logic to debug and maintain; (b) keeps checksumming as close to possible to data generation and use. This patch computes checksums after data has been buffered, and validates them before it is buffered. We sometimes use large buffers and would like to guard against in-memory errors. The current checksum code catches a lot of such errors. So we should compute checksums after minimal buffering (just bytesPerChecksum, ideally) and validate them at the last possible moment (e.g., through the use of a small final buffer with a larger buffer behind it). I do not think this will significantly affect performance, and data integrity is a high priority.