Switch HTTP server class to our own

Devel::hdb::Server is just like Devel::hdb::Server, except that it keeps
the connection open until we're done sending the response.

Consider submitting a patch back to the author
Refactor handling of stepin, continue

Client now handles stepin and continue with a delayed response.
They immediately return control to the user program, but set up
a closure to return the current stack data when the debugger regains
control.