Reduce the size of the index table

[mactrem]

The index table for a planet size vector tileset (z0 - z14) has currently about 3.1 gb in size.
Options to reduce the size:

• Only one absolute offset per fragment is stored and every index entry holds only the tile size (4 bytes). The client is responsible for resolving the absolute offset for the index entries of a fragment. This can reduce the index size for a planet vector tileset from 3gb to about 1.3gb
• Compress the index and add an additional fragment table to the archive which references the variable sized index fragments. The full fragment table can be fetched within the initial request