Implement per-page compression, page index

[benbjohnson]

This pull request makes several backward incompatible changes to the LTX file format and bumps the Version number to 2.

• Always compresses data (and there for removed the CompressLZ4 flag)
• Compresses each page individually rather than compressing the entire page block.
• Writes an index to the trailer of the file so that clients can maintain an index of page locations without downloading the entire file.

These changes will allow us to run a read replica VFS that pulls page data directly from S3 rather than restoring a full snapshot first.