Remove SelectValueTo

Benchmarks revealed that it is no longer performant enough to pull
its own wait. Using go_db_bench to copy JSON results to HTTP responses
it was ~20% *slower* for ~4BK responses and less than 10% faster for
+1MB responses.

The the performance problem was in io.CopyN / io.Copy. io.Copy
allocates a 32KB buffer if it doesn't have io.WriterTo or io.ReaderFrom
available. This extra alloc on every request was more expensive than
just reading the result into a string and writing it out to the response
body.

Tests indicated that if MsgReader implemented a custom Copy that used a
shared buffer it might have a few percent performance advantage. But the
additional complexity is not worth the performance gain.

README.md

@@ -114,14 +114,6 @@ data type is to set that OID's transcoder. See
 example_value_transcoder_test.go for an example of a custom transcoder for the
 PostgreSQL point type.
 
-### SelectValueTo
-
-There are some cases where Go is used as an HTTP server that is directly
-relaying single values from PostgreSQL (such as JSON or binary blobs).
-SelectValueTo copies the single returned value directly from PostgreSQL to a
-io.Writer. This can be faster than SelectValue then write especially when the
-values are at least many KB in size.
-
 ### Null Mapping
 
 As pgx uses interface{} for all values SQL nulls are mapped to nil. This