Allow exponential backoff to configure a once-off initial delay

We want to be able to configure a once-off delay before starting exponential increases with a low initial delay.

This would be used for bridge descriptors, client authority bootstrap, and test networks.

Follow-up to legacy/trac#20499 (moved), legacy/trac#20597 (moved), and legacy/trac#20534 (moved).

changed milestone to %Tor: unspecified in legacy/trac

added component::core tor/tor in Legacy / Trac milestone::Tor: unspecified in Legacy / Trac parent::20534 in Legacy / Trac points::1 in Legacy / Trac priority::medium in Legacy / Trac resolution::worksforme in Legacy / Trac severity::normal in Legacy / Trac status::closed in Legacy / Trac triaged-out-20170116 in Legacy / Trac type::enhancement in Legacy / Trac version::tor 0.2.9.1-alpha in Legacy / Trac labels

Trac:
Points: N/A to 1

batch modify: I think these are "enhancement", though I could be wrong about some.

Trac:
Type: defect to enhancement

Trac:
Keywords: N/A deleted, triaged-out-20160116 added
Milestone: Tor: 0.3.0.x-final to Tor: unspecified

Trac:
Keywords: triaged-out-20160116 deleted, triaged-out-20170116 added

This is already how the code is written, but it doesn't work in most cases because we don't reset the download status before we use it, see legacy/trac#17750 (moved).

Trac:
Resolution: N/A to worksforme
Status: new to closed

closed

changed time estimate to 8h

mentioned in issue legacy/trac#20606 (moved)

mentioned in issue legacy/trac#22400 (moved)

moved from legacy/trac#20604 (moved)

added Feature label and removed 1 deleted label

removed 1 deleted label