FileHistory regressions caused by history data sampling related optimizations

#1092 (closed) caused regressions for the time series charts:

• when adding new chart items, given manual scaling options do not take immediate effect but require reopening the time series chart editor or reconfiguring the item
• items added to charts do not even show up in the chart after adding them

Both problems seem to be caused by race conditions that followed the addition of the getItems/getItemManager caching logic. The file system watcher task was slow to react and some desirable synchronization was missing.

• Change getItems/getItemManager caching to be used only when explicitly asked to through a new function FileHistory.itemCaching(boolean b)
• Change the FS watcher thread to run continuously in the background instead of 100ms polling for faster response.