Add "multi-get" accessor

[cowtowncoder]

Sometimes it makes sense to get more than one entry, up to specified maximum, with one call. Main benefit is that synchronization is only done once, and the obvious downside is that multiple byte result byte arrays need to be allocated.

Although variants could be added to allow timed waits and such it seems best to start with a simple call that does NOT block (if none available, return empty array), and which takes single count argument that specifies maximum number of entries to return.