feat(server): add initial window builder methods that take self by-val

[seanmonstar]

The current Builder methods http2_initial_stream_window_size and
http2_initial_connection_window_size take &mut self, while every
other builder method takes self. That breaks up the chaining of
options.

This patch adds two methods that configure the same option, but take
self instead, and have an underscore suffix (so,
http2_initial_stream_window_size_).

cc #1814

[seanmonstar]

@sfackler any thoughts on whether the previous methods should get #[deprecated] attributes? Or is hiding them from docs enough?

[sfackler]

I don't feel super strongly about deprecation. There's nothing wrong with using the old methods...