fix: lower initial backoff to 100ms (from 1s)

1s is quite long, esp. when users expect rather low-latency operations.
It's also a bit of a pain for our tests because a single error (like
`NotAvailable`) can delay tests by a whole second easily leading to
timeouts.

src/backoff.rs

@@ -14,7 +14,7 @@ pub struct BackoffConfig {
 impl Default for BackoffConfig {
     fn default() -> Self {
         Self {
-            init_backoff: Duration::from_secs(1),
+            init_backoff: Duration::from_millis(100),
             max_backoff: Duration::from_secs(500),
             base: 3.,
         }