WIKI-740 WikiServiceImpl.getPageById invocation causes slowness

Fix description:
* Problem analysis In the same HttpRequest, same wikipage is searched from JCR many times
* How is the problem fixed?
Using a cache to store wiki page by Phase (Process Render, Process Action, Process Decode).
After one of these phases completing, cache is cleaned
WIKI-743: Slowness when checking permissions: Utils.canModifyPagePermission

Fix description: Avoid redundant call session.save() when getting Wiki object