Increase default size for ModuleDescriptorMemoryCache

Increase the default size of module descriptor memory cache from 150 to 1024.
There are some speedup gain when the dependency graph is large.

Related to sbt/ivy#4, increasing the cache size should reduce the IO overhead during Ivy resolution. Compared to cached `IvyNode#getDependencies` available on 2.4.x-sbt branch, the increasing ModuleDescriptorMemoryCache to 1024 shortened "large-graph" project's resolution from 12±1s to 11±1s. In the wild, I have a project with speedup from 14±5s to 13±1s.