Uses a couple of extra threads to work around the lack of cross-process
boxed trait objects.
r? @nnethercote
Source-Repo: https://github.com/servo/servo
Source-Revision: f778e0eecf7cd8a2b870d18c3c305ff10d6b1894
A rebuild after touching components/profile/mem.rs now takes 48 seconds (and
only rebuilds `profile` and `servo`) which is much lower than it used to be.
In comparison, a rebuild after touching components/profile_traits/mem.rs takes
294 seconds and rebuilds many more crates.
This change also removes some unnecessary crate dependencies in `net` and
`net_traits`.
Source-Repo: https://github.com/servo/servo
Source-Revision: 77f653da2c4120ea7ac1a946d97fc70059d513d4
