memstore is not threadsafe

[dpavsrtrl]

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.

[iddan]

Do you consider working on a consistent concurrent memstore?

[dennwc]

Still relevant. Will need to implement it at some point.

[dpavsrtrl]

Ok, I'm sorry. Don't have time currently.