Updates #7475
The Java implementation has a 128KB buffer and a message must be emitted before that is used. #7475 therefore limits the message size to 128KB. But up to 256 bytes are written to the buffer in each call. This means we must emit a message before shorter than 128KB.
Therefore we change the limit to 128KB minus 256 bytes.
Queue output items and reuse them.
Remove the unneeded type system in sql and just use the Go type system.
In best case this is more than an order of magnitude speedup:
```
BenchmarkSelectAll_1M-12 1 1841049400 ns/op 274299728 B/op 4198522 allocs/op
BenchmarkSelectAll_1M-12 14 84833400 ns/op 169228346 B/op 3146541 allocs/op
```
- Staging buffer is flushed every 500ms. In cases where the result
records are slowly generated (e.g. when a where condition
matches very few records), this change causes the server to send
results even though the staging buffer is not full.
- Refactor messageWriter code to use simpler channel based
co-ordination instead of atomic variables.
Batching records into a single SQL Select message in the response
leads to significant speed up as the message header overhead is made
negligible.
This change leads to a speed up of 3-5x for queries that select many
small records.
