/spanio, /spanblock, /spansched, /spansyscall provide
the pprof-style summary of span execution's
io, block, scheduling, syscall latency distributions
respectively.

The computation logic for /io, /block, /sched, /syscall
analysis was refactored and extended for reuse in these
new types of analysis. Upon the analysis query, we create
a map of goroutine id to time intervals based on the query
parameter, that represents the interesting time intervals
of matching goroutines. Only the events from the matching
goroutines that fall into the intervals are considered
in the pprof computation.

The new endpoints are not yet hooked into other span
analysis page (e.g. /userspan) yet.

Change-Id: I80c3396e45a2d6631758710de67d132e5832c7ce
Reviewed-on: https://go-review.googlesource.com/105822Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>