Major improvements to Voxel Packet "packing" and compression

- Improves how Voxel Packets are "packed" resulting in about 30% better utilization of 1500 byte packets
- Fixed a bug that caused us to include subtrees for LOD cut voxels that were determined to have their colors sent (another 20%-30% savings)
- Implements support for "Compressed Voxel Packets" - sometimes saves 15%-60% depending on content
- got rid of a whole bunch of old menu items and settings we don't need anymore. 
- consolidates PACKET_TYPE_VOXEL_DATA and PACKET_TYPE_VOXEL_DATA_MONOCHROME into single packet type
- adds packet headers to PACKET_TYPE_VOXEL_DATA with: sequence number, sent time, flags to indicate monochrome/color and compressed/uncompressed.
- cleans up how PACKET_TYPE_VOXEL_DATA is "packed" by using a new class VoxelPacketData which tracks statistics about how bytes in the packets are used.
- adds more server side stats for tracking packet packing efficacy
- cleans up client side voxel stats dialog box to display actual content about voxel stats from multiple servers.
- adds client side preference setting for "Max PPS" for incoming voxel packets.
- get rid of PACKET_TYPE_Z_COMMAND
- clean up "const"-ness in some helper functions that should have been const