Summary: Currently it's easy to use a ton of memory with many small
OptimisticTransactionDB instances, because each one by default allocates
(by default) a million mutexes (40 bytes each on my compiler) for
validating transactions. It even puts a lot of pressure on the allocator
by allocating each one individually!

In this change:
* Create a new object and option that enables sharing these buckets of
mutexes between instances. This is generally good for load balancing
potential contention as various DBs become hotter or colder with txn
writes. About the only cases where this sharing wouldn't make sense
(e.g. each DB usually written by one thread) are cases that would be
better off with OccValidationPolicy::kValidateSerial which doesn't use
the buckets anyway.
* Allocate the mutexes in a contiguous array, for efficiency
* Add an option to ensure the mutexes are cache-aligned. In several
other places we use cache-aligned mutexes but OptimisticTransactionDB
historically does not. It should be a space-time trade-off the user can
choose.
* Provide some visibility into the memory used by the mutex buckets with
an ApproximateMemoryUsage() function (also used in unit testing)
* Share code with other users of "striped" mutexes, appropriate
refactoring for customization & efficiency (e.g. using FastRange instead
of modulus)

Test Plan: unit tests added. Ran sized-up versions of stress test in unit
test, including a before-and-after performance test showing no
consistent difference. (NOTE: OptimisticTransactionDB not currently covered
by db_stress!)

…n_memory