net/http/pprof: allow "seconds" parameters to most profiles

golang.org/cl/147598 added the support for delta computation for mutex
and block profiles. In fact, this delta computation makes sense for
other types of profiles.

For example, /debug/pprof/allocs?seconds=x will provide how much allocation
was made during the specified period. /debug/pprof/goroutine?seconds=x will
provide the changes in the list of goroutines. This also makes sense for
custom profiles.

Update #23401
Update google/pprof#526

Change-Id: I45e9073eb001ea5b3f3d16e5a57f635193610656
Reviewed-on: https://go-review.googlesource.com/c/go/+/229537


Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>