Fix riak_core_util:pmap/2 infinite stall

This commit changes riak_core_util:pmap/2 to use spawn_link rather
than spawn to create the asynchronous processes. Without this change,
pmap/2 can stall forever if any of these processes crashes -- eg. by
using a map function that generates an error.

This commit also fixes the function -spec as well as optimizes the
final stage of the pmap by using a list comprehension rather than
lists:unzip (this is faster and generates less garbage).
Add pmap stall unit test
Merge pull request #602 from basho/bugfix/pmap-stall

Fix riak_core_util:pmap/2 infinite stall

Reviewed-by: engelsanchez