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, as data can be shared between tasks, so only a small set of files can be merged.

Note because of a need to maintain backward compatibility, 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 WDs -> fsnotify