Shmem4: a set of small fixes to shmem implementation

This is a set of small fixes and improvements of the shared memory/mmap support.

Other than trivial one-liner fixes, there are two major changes:

- update memmgr and its library to handle update failure better (see log for commit 662b2a31be29be7b226d82f81d7a00568e35a737)
- "pre-load" zone data diff when applying outstanding diffs on loading to memory for improving stability (see commit 965d075b617f825db27942c0d3ffcec6fc0e908d)