Combine adjacent mappings even if offsets are unavailable
Some profile handlers will split mappings into multiple adjacent
entries. This interferes with pprof when overriding the main binary.
Currently there is code that attempts to combine these mappings, but
it fails if the mapping offsets aren't available. Add a check to
combine them in that situation.
Add field for profile sources to indicate what is the primary
sample value, which visualizers should display by default.
Previously there was a convention for pprof to use the last
sample_index by default, this provides more flexibility.