Add statistics on time spent on crypto operations

We'd like to find out how much of Tor's time is spent on which crypto operations, and we'd want to answer this question over time.

These statistics would help us understand the implications of other design shifts -- like directory guards, where one of the desired side effects is a reduced number of TLS handshakes for relays because clients handshake with fewer relays. Another benefit could be that we'd have better data on hardware requirements for the Torouter and similar projects, in particular crypto operations performed by an average relay or bridge.

Probably the easiest way to achieve the "answer over time" part would be to have relays/bridges add a new line to their extra-info descriptors, and later look at metrics archives to see how things changed over time. There should not be too big privacy implications if we make the granularity sufficiently big.

We might want to make this feature optional, since it takes time to check the time. (Gotta find out how many nanoseconds for a hi-resolution timers vs how many nanoseconds for encrypting a cell. That would be part of the design.)