Use strings to identify section types in the on-disk repo format.

Use strings to identify section types in the on-disk repo format.

Previously, a given razor file type had a fixed number of sections in a
fixed order, identified by an integer type. Now, sections are identified
by a named string (stored in a string pool after the section lists).

This will allow for razor files to contain arbitrary sections.

For bonus points, also drop the 4k section alignment and change the
magic byte string to "RZDB".