sumdb: restore Go 1.12 support

Change-Id: I4711da945699bbe9416a804f6ec28ca95eb4bb7c
Reviewed-on: https://go-review.googlesource.com/c/exp/+/174397
Run-TryBot: Filippo Valsorda <[email protected]>
Reviewed-by: Katie Hockman <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

sumdb/internal/sumweb: simplify code, clarify comments

Followup work from CL 172964 review comments.
(That CL was in a different directory and is too hard to get back to.)

- Simplify to one cache for Conn.ReadTile.
- Replace Client.GetURL with Client.ReadRemote,
  which takes only a path, not a full URL.
  It works better for the client to take care of URL mapping.
- Rename FindKey to Lookup throughout (matches URL).
- Apply !-decoding in Handler.

Change-Id: Ib004ebb1b608d9ee2f8bdf768628acdd5b985158
Reviewed-on: https://go-review.googlesource.com/c/exp/+/173945
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>