* Implement missing pieces for connection backoff.
Spec can be found here:
https://github.com/grpc/grpc/blob/master/doc/connection-backoff.md
Summary of changes:
* Added a new type (marked experimental), ConnectParams, which contains
the knobs defined in the spec (except for minConnectTimeout).
* Added a new API (marked experimental), WithConnectParams() to return a
DialOption to dial with the provided parameters.
* Added new fields to the implementation of the exponential backoff in
internal/backoff which mirror the ones in ConnectParams.
* Marked existing APIs WithBackoffMaxDelay() and WithBackoffConfig() as
deprecated.
* Added a default exponential backoff implementation, for easy use of
internal callers.
Added a new backoff package which defines the backoff configuration
options, and is used by both the grpc package and the internal/backoff
package. This allows us to have all backoff related options in a
separate package.
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