Processing limit is reduced for SSE requests

[mkauf]

I have debugged a problem with SSE (server sent events). The internal DoS protection of mod_h2 has a bug, it evaluates stream resets twice. It tolerates the reset the first time. It doesn't tolerate it the second time.

When a stream reset is received, this happens:

1. From on_frame_recv_cb(), h2_mplx_c1_client_rst() is called. reset_is_acceptable() returns 1. (m_be_annoyed() is not called.)
2. From on_stream_close_cb(), h2_stream_rst() is called, this sets stream->rst_error. Then, h2_mplx_c1_client_rst() is called again, this time reset_is_acceptable() returns 0 because stream->rst_error is set. m_be_annoyed() is called.