Cache all cube image data

[markccchiang]

Add an option and allow the backend to cache all cube image data in order to improve performances for CASA, FITS, and MIRIAD images. Users can set the number of reserved memory while starting the backend. The backend will cache all cube image data if the total amount of its memory space is under the reserved memory. Theoretically, it can improve performances especially when accessing z-direction data, like spectral profiles, cube histograms, and PV image generation. The image moments calculations should be improved too. But it will be left and done in a separate issue and PR.