Optimize constructor time for tiledb-cloud case

[johnkerl]

Analysis for various read & write operations on tiledb://... URIs reveals that:

• Core-to-REST-server requests appear in things like tiledb.object_type, tiledb.group.Group, etc.
• These have server-side latency
• There are server-side optimizations we can do, certainly, but also on the client side we can batch and cache:
  • Getting group-element name-to-URI mappings takes about half a second per request whether we ask for one group member at a time, or for all group elements at once.
  • Once we've gotten a name-to-URI mapping -- or a construction of an object which contains such -- we can cache the result to avoid subsequent, redundant requests (with, of course, cache-invalidation logic in the right places)

Later note: more in #223.

[aaronwolen]

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