Implement exponential backoff in egress RPC client retry logic

[biglittlebigben]

This uses exponential increases for both the timeout and backoff parameters. The formula is for the increase is:

base * 2 ^ (attempt - 1) * rand[1,2)

This means that for the different attempts, we get:

1: base * [1-2)
2: base * [2-4)
3: base * [4-8)
4: base * [8-16)
...

[biglittlebigben]

Thinking about this again, there is no reason to have a random component to the timeout parameter. Will change this.