Image field merging

The number of image files has a penalty over migration process speed. Thus we initiated the _merging_ procedure which is aimed to reduce the number of image files by merging files one into another. Unfortunately we can't just merge everything into one big file because data can be shared between tasks thus only small set of files can be merged.

Note because of backward compatibility requirement we need to support both merged and separate images for a while.

Here is a list of merged data so far:

• vma_entry -> mm_entry::vmas
• rlimit_entry -> core_entry::task_rlimits (sent series)
• posix_timer_entry -> core_entry::task_posix_timers (sent series)
• fsnotify WD-s -> fsnotify