Small tweak to remove a connection from a context when it is closed.
Small tweak to remove a connection from a context when it is closed.

* context.c:
  (GET_CONN): new macro to access a connection within a context
  (open_connections): use GET_CONN rather than manual indexing
  (serf_connection_close): use GET_CONN rather than manual indexing. after
    closing the bits in the connection, remove it from the context. have
    the function return APR_NOTFOUND if the connection is unknown.

jandre/serf,jandre/serf,jandre/serf,jandre/serf