Add protocol for iterating keys in a store

[csm]

Introduces a new protocol PKeyIterable, which defines the method -keys that returns a channel yielding all toplevel keys in the store, in sorted order. Adds implementations of this for memory and filestore.

This adds a function konserve.core/keys, which might not be the best name since it overrides clojure.core/keys. Alternate name suggestions are welcomed.

• src/konserve/core.cljc (keys): new function.
• src/konserve/filestore.clj (list-keys): do I/O in a thread. (FileSystemStore): add PKeyIterable protocol.
• src/konserve/memory.cljc (MemoryStore): add PKeyIterable protocol. (new-mem-store): force state to be a sorted-map.
• src/konserve/protocols.cljc: Add PKeyIterable protocol.

[csm]

I was trying to think of ways to do node GC in hitchhiker-tree, and at least some way to iterate keys in the store is a starting point. There might be other approaches, and the keys output might not need to be sorted.