Limit maximum size ATS can read from net at a time in SSL read loop

[maskit]

Limit maximum size ATS can read from net at a time in SSL read loop. Hopefully avoids timeouts during POST.

[maskit]

I don't think the value should be hardcoded. Using loop count is an option too. The point is that we need to break out from this loop somehow without any errors at times.

[scw00]

IMOH, there is a little problem .

After discussed with @oknet, this logic seems to be the same as #629.

We only fill one block in the original logic and then call user

block_write_avail = b->write_avail();

After #629
We always append block if ntodo not 0.

[oknet]

The NetHandler is designed to:

1. Only read() one block of data and then call Continuation ( if watermark set to 0 ).
2. The Continuation consume data and reenable the read VIO and return.
3. The vc->read.triggered is set to 0 if EAGAIN returned from read()
4. Go to step 1 if next NetVC not null and vc->read.triggered is 1.

After #629 merged, the ssl_read_from_net() will read and attach IOBufferBlock(s) until EAGAIN.
It makes NetHandler stall on reading loop and cannot going to the sending loop.

[oknet]

Whether or not add a new block , It is controlled by the watermark of MIOBuffer.
Just break out here if buf.write_avail() less than 0.

[maskit]

Thanks for the comments. Discussion will be continued on #2248.