Add backoff system implementation

[petar]

Highlights:

• Abstract definition of a backoff strategy (which is more expressive than the one in go-libp2p-discovery), e.g. it allows handling backoff bursts
• Add "backoff policy" which is a factory for backoff strategies. BackoffPolicy also offers a more idiomatic way of creating factories in Go, by utilizing the type system
• Add implementation of hierarchical backoff timers
• Add implementation of the shared backoffs interface (in go-libp2p-core/backoff). This is the entrypoint for all users of this library.

This PR depends on: libp2p/go-libp2p-core#127

[willscott]

remove

[willscott]

a task like this should probably run within a context so that it can be completed on shutdown.

[willscott]

a test with two children subtimers active in different go-routines would be potentially useful to find races.