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`.
> This comment is confusing as there might be a connection by the time this is invoked.

 Just to note that this doesn't happen in the current code base as per "maximum one repl per connection rule" but might be useful in the future.