cmd/trace: support splitting in goroutine view

[prattmic]

For the main trace view, we support splitting the trace into smaller time chunks. e.g., /trace?start=966781&end=1937169. This is used to split the trace into small chunks that will (hopefully) prevent the viewer from choking on too much data.

View trace (0s-325.884822ms)
View trace (325.884822ms-686.872205ms)
View trace (686.866981ms-1.054600408s)
View trace (1.054600408s-1.386457684s)
View trace (1.386457684s-1.730625581s)
...

The goroutines view (/trace?goid=1) does not provide links to a split view. In theory, this can be done manually by adding start and end parameters to the URL. In my experience, this works fine if start=0 (it will provide a trace truncated at the end time). However, any start time other than 0 results in an empty trace.

cc @mknyszek