MINOR: ssl: handshake optim for long certificate chains.

MINOR: ssl: handshake optim for long certificate chains.

Suggested on the mailing list by Ilya Grigorik and greatly inspired
from Nginx code: we try to dynamicaly rise the output buffer size from
4k to 16k during the handshake to reduce the number of round trips.
This is mostly beneficial when initcwnd==10.

Ilya's tests confirm the gain and show a handshake time divided by 3 :

before:
   http://www.webpagetest.org/result/140116_VW_3bd95a5cfb7e667498ef13b59639b9bf/2/details/
after:
   http://www.webpagetest.org/result/140201_2X_03511ec63344f442b81c24d2bf39f59d/3/details/