Extend gops trace to allow duration parameter

[glibsm]

gops trace :port 10m will now run a trace for 10 minutes,
as opposed to always doing 5 seconds.

For backwards compatibility both server and the client
default to 5s if nothing is specified.

[glibsm]

cc @tklauser

[tklauser]

Nice, thanks for the PR! The code changes LGTM. Could you please update the gops trace usage example in README.md as well?

[tklauser]

I've tested this with the following combinations using examples/hello as the agent:

1. new agent, new gops binary: gops trace <pid> 10s -> works as expected
2. new agent, old gops binary: gops trace <pid> -> gops hangs indefinitely
3. old agent, new gops binary
   a. without duration: gops trace <pid> -> gops exits with read tcp 127.0.0.1:53968->127.0.0.1:38565: read: connection reset by peer without displaying a trace
   b. with duration: gops trace <pid> 10s -> same as 3a above.

(where "new" means built from this PR and old means built from current master)

It looks like this change is not backwards compatible. Do you think we can modify it in a way that cases 2 and 3a/b would work as well?