memstore is not threadsafe

Tried running memstore example with -race and and got errors.

After submitting PR #780 in an attempt to make memstore threadsafe, discovered that it can be threadsafe but not consistent for readers when a writer is updating it (by deleting, adding).

Now I believe that the issue is not that current memstore is not threadsafe, but there is no concurrent memstore.

Start with the memstore example and run it with -race to reproduce the reported errors. Review PR #780 and the issue discussion to understand the distinction between thread safety and consistent concurrent reads; done would require a defined concurrent memstore design, which the issue does not specify.

Written by the indexing model from the issue text.