Return uint64 from exponentBase2 function (#753)

A 32bit binary could not be built because `1 << 62` would overflow the
return `uint` type, as it is an architecture dependent type.

Closes #752

interceptors/retry/backoff.go

@@ -27,7 +27,7 @@ func jitterUp(duration time.Duration, jitter float64) time.Duration {
 
 // exponentBase2 computes 2^(a-1) where a >= 1. If a is 0, the result is 1.
 // if a is greater than 62, the result is 2^62 to avoid overflowing int64
-func exponentBase2(a uint) uint {
+func exponentBase2(a uint) uint64 {
 	if a == 0 {
 		return 1
 	}