Normalize addresses before sending

[brancz]

We currently do something fairly odd, which is that we add a non-standard comment into the pprof message. We do this in order to perform server-side normalization of addresses. However, pprof actually has a mechanism to signal that addresses have already been adjusted to be machine independent, by setting mapping start, length and offset to 0.

What does that mean? If we set mapping start, length, and offset to 0, and adjust the address (addr - (start - offset)) on the client already, then we both maintain support with pprof tooling and can get rid of the non-standard comment.

@umanwizard