Add debug log to doFetchDependencies

This is for resolution performance analysis. See sbt/ivy#4
Cache IvyNode#getDependencies

IvyNode#getDependencies is one of the hot spots frequently called during resolution.
Adding caching at this level is not as invasive, and has a nice speedup.
Using large-graph sample project, 17s resolution improved to 10s.
Add generics comment
Merge pull request #5 from sbt/wip/cache-getdependencies

Cache IvyNode#getdependencies