Redefine backoff strategy for retry middleware

[ksysoev]

Current implementation of retry middleware use equals intervals between retries. It may be useful to have possibility to redefine policy for retry intervals.

probably we can accept optional argument with a function that will accept 2 parameters with current retry counter and initial retry interval, based on these parameters function should calculate interval that needs to be awaited.

[shan-96]

@ksysoev can I take this over? Also I think I don't need to wait for #94 to finish to begin this.

[ksysoev]

@shan-96 sure, yes I think #94 can be implemented as separate parameter