allow DzlCounter to grow map region for larger numbers of counters
@chergert
Submitted by Christian Hergert Link to original bug (#749280)
Description
Right now, we don't play well with counters that cause us to overlap our allocated page size.
Growing the shm_open()'d file-descriptor is easy (using ftruncate to zero things). However, we can't move the mmap regions at runtime, or we could cause faults by running counters.
Therefore, each additional page needs to be allocated individually, and that means we will not have contiguous pages.
To work around that issue, we can just avoid adding counters at locations that would cause them to overlap pages. The local and remove implementations need to be smart enough to avoid this. (Actually, the reader probably doesn't need any help since it does not refresh when new counters are added).