sbe: messages are found by template id alone, so two schemas sharing one collide (protowire-go#256, #258, #270) #134

A template id is unique only within a schema. This port keys messages by template id alone and never compares a header's schema id. protowire-go fixed this in three commits: trendvidia/protowire-go#256 and #258 (issue #253, decided by the owner), and #270 (issue #257, owner chose option 1; spec trendvidia/protowire#571).

What C++ does now (by reading main a632c6e, not measured):

• Codec::RegisterMessage (src/sbe/codec.cc) does by_id_[owned->template_id] = …. A second message with the same (sbe.template_id) silently replaces the first, in the same schema or in another one.
• Codec::NewView looks up by_id_ by template id alone. It finds the last message registered and reads the other schema's bytes as that message.
• Codec::UnmarshalInto (src/sbe/unmarshal.cc) checks the header's template id against the target, but not its schema id (header bytes 4-5). Schema 1's bytes for template 7 decode as schema 2's template 7.

Go's behaviour, to mirror:

• Codec::New refuses two different messages with one (sbe.template_id) in one schema: sbe: messages %s and %s share (sbe.template_id) = %d in schema %d; it must be unique within a schema. The same message registered twice, as when a file is passed twice, is not a duplicate. Two schemas that share a template id still bind.
• by_id_ is keyed by (schema id, template id), and NewView reads the schema id from header bytes 4-5. An unknown pair is refused with sbe: unknown template ID %d in schema %d, even if another schema has that template id.
• Unmarshal and UnmarshalDescriptor refuse a schema id mismatch: sbe: schema ID mismatch: got %d, want %d for template %d. The version is not checked.
• Tests ported from Go's encoding/sbe/template_id_test.go, plus TestView_BlockLengthTooSmall's header now setting a schema id.

This narrows what decodes. A caller that builds headers by hand must now set the schema id. The CHANGELOG should say so, as Go's does.

Note: #124 (stale-descriptor diagnosis) also edits RegisterMessage, so this work should stack on it or follow its merge.