Add support for relative paths in table metadata

[rdblue]

Metadata currently tracks all paths using the full path. This is costly when not using compression (like the current metadata files) and doesn't allow a table to be easily relocated. The format should support relative paths with respect to the table root, or full paths.

[julienledem]

where would the table root path be stored?

[rdblue]

There's already a location in the metadata that we use to build paths. We could just use that.