Set connection transport buffer for active socket

[kennethito]

fixes #362

[whatyouhide]

@kennethito thank you so much for the PR! However, there's one thing we need to tweak. Right now, there are socket options that we let the user customize (say, the SSL opts) and options that we force (like active: :once). I think this buffer size option should be one that we let the user customize, falling back to a default of 1_000_000 as seen in this PR. Thoughts?

[kennethito]

@whatyouhide Ah, didn't notice the existing transport options. Updated!

[whatyouhide]

Perfect! Can we mention this in the documentation for the transport options? Then, we're good to go 💯

[kennethito]

Updated with documentation!