OpenBSD does not have a unified buffer cache, which does not play nicely
with LMDB's use of a combination of a read-only mmap() and "regular"
writes. One workaround is to set the MDB_WRITEMAP flag when creating
LMDB environments on OpenBSD. This causes the LMDB library to mmap()
its databases in read-write mode and perform stores by writing to
addresses mapped in such a way instead of using "regular" writes.
To avoid repeating the same set of flags over and over in different
places, #define them in a header file.
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