Make the first exp of retry with backoff be 0

[lahiguera]

Context:

Previously the first exponent used to calculate the first waiting
time when retrying using the expoential backoff algorithm with
full jitter was 1, i.e. 2**1 was used instead of 2**0. This made
the first waiting time longer (only potentially because of jitter)
than what it's supposed to be.

Also previously the tests in retry_test.go that relied on
patternTimer were not actually testing anything, since the Run
method of such struct prevented the recording of the waiting times
because prevExecution was always equal to time.Time{}.

This PR fixes the issues mentioned above.