Make TCP buffer size configurable

[michaelklishin]

Proposed Changes

This PR makes TCP buffer size and credit flow defaults configurable, as requested in #459.

Types of Changes

• Bug fix
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

Closes #459.

[lukebakken]

We could add buffer as well, since this code selects the max of sndbuf, recbuf and buffer to use for the socket buffer size - what do you think @michaelklishin ?