ztoc versioning needs clarification

[Kern--]

Also, we have gzip_zinfo versions (

soci-snapshotter/compression/gzip_zinfo.h

Lines 56 to 57 in 3b2cc11

	#define ZINFO_VERSION_ONE 1
	#define ZINFO_VERSION_TWO 2

), and in the future maybe zstd_zinfo version(s).

How do we plan to track these versions altogether? a zinfo version change will possibly change zinfo bytes, which changes ztoc. (e.g. a 0.9 ztoc with version1 gzip_zinfo != a 0.9 ztoc with version2 gzip_zinfo, but to user they're the same 0.9 ztoc).

Originally posted by @djdongjin in #485 (comment)

[Kern--]

Another example of clarification needed: #853 (comment)

FileMetadata.XAttrs in the flatbuffer actually contains PAX records. Should/Could a major version change fix the naming?