Use repl buffer as connection buffer

This is a first comit on the road to replanting cider-repl on top of comint. 

 These are the related code adjustments that were (mostly) necessary for the patch:

 - new functions `nrepl-start-client-process' and `nrepl-start-server-process'.
 - `cider-init-repl-buffer' and `cider-create-repl-buffer' have clear cut
    separation of functionality
 - `nrepl-make-buffer-name` and all `cider-create-xxx-buffer-name` take explicit
    dir, port and host variables.
 - increase `nrepl-decode-timeout` to 0.025 for greater reliability
 - merge `nrepl--handle-process-output` into client process filter
 - remove `nrepl-connect' in favor of `nrepl-start-client-process'.