Fix thrift_buffered_transport leak in erl bindings

Fix thrift_buffered_transport leak in erl bindings

Summary:
  - Was leaking processes when the client cleanly disconnected since the
    thrift_processor process was exiting with 'normal' Reason. Changed it
    to 'protocol_closed'

Test plan:
  - Telnet to thrift service port and close (^]close) before the read timeout
    elapses. Verify that erlang:system_info(process_count) does not increase
    after doing this multiple times.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@666430 13f79535-47bb-0310-9956-ffa450edef68