Support configurable client deadlines in call()

[TheJakeSchmidt]

Currently, RPC calls have a client-side deadline of 5 seconds, hard-coded in fetch() src/client.cc. We have some RPCs that take longer than that, that we'd like to call from R, so can this be made configurable from R?

[nfultz]

This seems like a pretty reasonable ask and straightforward to implement; I'll add it to the queue.