Provide a way to adjust the initial window size that does not also disable BDP estimation

[dfawley]

There are at least a couple options here:

1. Change the existing Dial/ServerOptions (e.g.) so they do not disabled BDP estimation

• Optionally, add another set of options that does disable BDP estimation.

2. Add new Dial/ServerOptions to set the size like the existing ones, but which do not also disable BDP esimation.

[dfawley]

Proposal:

1. Add new settings:

package grpc

func WithStaticStreamWindowSize(int32) DialOption
func WithStaticConnWindowSize(int32) DialOption

func StaticStreamWindowSize(int32) ServerOption
func StaticConnWindowSize(int32) ServerOption

These will behave the same as the current options.

2. After 2 releases, with a notice that the behavior is changing in the release notes, change the current options so they do not disable BDP estimation. Also add:

func WithInitialStreamWindowSize(int32) DialOption // just does "return WithInitialWindowSize(n)"

func InitialStreamWindowSize(int32) ServerOption // just does "return InitialWindowSize(n)"

And mark WithInitialWindowSize and InitialWindowSize as deprecated.

Rationale:

• The current options' names imply they only affect the starting window size, but that the window size is adjustable. Leaving the behavior the same, and adding new options that specify a starting size but adjust over time would be confusing for everyone.