tonic: Server::default(): Set TCP_NODELAY to true; improve docs

[evanj]

Motivation

I recently ran into problems where we accidentally "lost" the tcp_nodelay=true setting, due to refactoring some code to make it easier to test. This caused mysterious 40 ms latency increases. I hope this makes it less likely for others to make this mistake.

Solution

The documentation of the Server::tcp_nodelay() function said "Enabled by default", but that was only true when using Server::builder(). The default() method set this to false. To fix this:

• Change Server::default() to set tcp_nodelay: true.
• Change Server::builder() to just call Server::default().
• Add a test to verify the settings for nodelay and keepalive.
• Document the functions to note that the TCP settings are ignored when using serve_with_incoming. tcp_keepalive already had this note. I added the same documentation to tcp_nodelay and to serve_with_incoming so it is less likely to be missed.