lazy evaluation! actions that have mayExport = false will be put into buffer and executed later.
allow pages to be cached into DFS and loaded later to reduce network dependency
autoSave and autoCache both enabled by default. their default path pattern is:
root/backtrace_generated_dir/blockKey/timestamp.
ClientAction named back to action as package is enough to distinguish.
Container renamed to Block
All time parameter type change to Duration to allow more flexible definition
Page now have blockKey as part of UID to handle multiple page exported from a block action.
Everything in backtrace now extends Product to ensure they are case classes
Lookup is defined as a function from PageUID to path.
Minor refactoring for simplicity