Make contexts auto-registered, ensures userland contexts
and C API contexts are both dealt with on request shutdown.
Also brings contexts in keeping with streams which are already
auto-registered.
This breaks user-space filters (for the time being), and those
weird convert.* filters in ext/standard/filters.c
The filters stack has been separated into one chain for read and one chain
for write.
The user-space stream_filter_append() type functions currently only operate
on the read chain. They need extending to work with the write chain too.
