Remove context deadline checking in backOffContext (cenkalti#113)

* Remove ctx deadline check

* Add context error checking after stop

bump minimal go version to 1.13

add next interval to elapsed time before deciding to stop exponential…

… backoff cenkalti#95

Fix crash if nil passed to NewTickerWithTimer

Doc for function says that passing nil for the timer argument should
cause the default system timer to be used.  Ensure that that timer is
actually assigned.

do not panic when stopping nil timer; fix cenkalti#88

require minimum go 1.12

Merge branch 'justin-mp-v3' into v3

* justin-mp-v3:
  Return context error from RetryNotify when done

Merge branch 'Oppodelldog-v3' into v3; fix cenkalti#74

* Oppodelldog-v3:
  fix flaky ticker test
  unexport DefaultTimer
  refactor; move Timer to its own file
  provide custom timer in tests to speed up
  add NewTickerWithTimer function
  Abstract timer corresponding logic towards an interface to enable  user defined timer behavior. By default time.Timer is used as timer.

respect context in WithMaxTries; fix cenkalti#86

time.Until not available in go 1.7