reduce suggested receive_buffer_size

[mpavlov]

The current recommendation for receive_buffer_size is 3.125 * 2^23.

On macOS Sierra, the default max socket buffer size is 2^23.

$ sysctl kern.ipc.maxsockbuf
kern.ipc.maxsockbuf: 8388608

When running in macOS with the recommended receive_buffer_size, we get the following:

I, [2017-02-02T10:12:43.273220 #98870]  INFO -- FFWD::Plugin::Protobuf: Setting receive buffer size to 26214400
W, [2017-02-02T10:12:43.273296 #98870]  WARN -- FFWD::Plugin::Protobuf: Bind on udp://localhost:19091 failed, retry #1 in 10000s: No buffer space available - setsockopt
/Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/protocol/udp/bind.rb:72:in `block in initialize': undefined method `close' for #<FFWD::Plugin::Protobuf::InputUDP:0x007fbd54955b78> (NoMethodError)
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/retrier.rb:58:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/retrier.rb:58:in `block in try_block'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/retrier.rb:57:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/retrier.rb:57:in `rescue in try_block'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/retrier.rb:53:in `try_block'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/retrier.rb:37:in `block in initialize'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:83:in `block in depend_on'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:83:in `block in depend_on'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/lifecycle.rb:51:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/core.rb:132:in `block in run'
	from /Library/Ruby/Gems/2.0.0/gems/eventmachine-1.0.4/lib/eventmachine.rb:187:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/eventmachine-1.0.4/lib/eventmachine.rb:187:in `run_machine'
	from /Library/Ruby/Gems/2.0.0/gems/eventmachine-1.0.4/lib/eventmachine.rb:187:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd/core.rb:128:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/lib/ffwd.rb:360:in `main'
	from /Library/Ruby/Gems/2.0.0/gems/ffwd-0.4.2/bin/ffwd:9:in `<top (required)>'
	from /usr/local/bin/ffwd:23:in `load'
	from /usr/local/bin/ffwd:23:in `<main>'

Unless there's a specific reason for picking 3.125 * 2^23, we could change our recommendation to something smaller than 2^23.