Lower RAM usage for tiles

This lowers RAM usage by an extreme amount. On windows, roughly 1/2 of
the RAM is used now, and, with mono, roughly 1/4-1/5 of the RAM is used
now, bringing servers from 1GB+ down to a manageable 350MB for a large
world.

Unfortunately, this breaks compatibility with all existing tile-based
plugins, but the gains are more than the losses of having to recode the
existing tile-based plugins.
Clean out client-based classes
Remove ServerSock.writeBuffer, lower messageBuffer.readBuffer

Because the server block copies ServerSock.readBuffer into
messageBuffer.readBuffer and the first buffer is 1024 bytes, only 1024
bytes are needed for messageBuffer.readBuffer instead of 65535, which
leads to a noticeable improvement in RAM usage.