Revert r854 and replace with a better strategy by marking a pollset as dirty when needed and only updating the pollset when we next run the context.
Revert r854 and replace with a better strategy by marking a pollset as dirty
when needed and only updating the pollset when we next run the context.

* context.c
  (serf_context_t): Add flag to indicate we have a dirty pollset.
  (serf_connection_t): Indicate our connection needs its pollset entry updated
  (open_connections): Flag our pollset and new connection as dirty
  (write_to_connection, read_from_connection): When we're exhausted, change
  our pollset flag.
  (check_dirty_pollsets): Walk our connection list if we have a dirty pollset.
  (serf_context_run): Call check_dirty_pollsets after openning connections.
  (serf_request_deliver): Always update the pollset flag (the root cause of
  the problem identified in r854).

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