Don't use managedCache for library stamps
Don't use managedCache for library stamps

Using the managedCached introduced an unintended performance regression
because it ensured that we always computed the hash of each jar on the
dependency classpath. The backing ReadStamps only computes the stamp if
the timestamp of the jar has changed.

mdedetrich/sbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,mdedetrich/sbt,xuwei-k/xsbt,xuwei-k/xsbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt