broadcast discovery should create a timeout context used by the client

[ripienaar]

When doing discovery the supplied context is used verbatim:

https://github.com/choria-io/go-client/blob/3abc5542c103b7a2c1990c099097dc756c0de4dc/discovery/broadcast/broadcast.go#L85

This means that if the initial connection fails or whatever - it's going to be stuck here forever rather than discovery timeout period. Probably we need a initial connect timeout rather than a whole timeout over the entire discovery but for now lets just wrap the whole client request in a context and give it timeout+2