This class implements a shared memory key-value store that fits into a single
memory mapped segment. All of the runtime data for its instances are stored in
the shared memory region, which means that memory overhead for each instance
in each process is only a few bytes.
Importantly, the key and value strings returned by this class are also
pointers into the shared memory region, which means that once an instance is
created, its memory cannot be unmapped until process shutdown.
For the uses I intend to put it to, this is a reasonable constraint. If we
need to use it for shorter-lived maps in the future, we can add an option to
return non-literal dependent strings that will be copied if they need to be
kept alive long term.
MozReview-Commit-ID: 5BwAaDsb7HS
--HG--
extra : rebase_source : b472fe628018f88a2c4d6b3de4b7143aeca55e14
extra : absorb_source : 5cdeb568cfd2b4a5a767191402e699e61e653b3b
