Buffer is flushed🚽 after every single Redis query

[Al2Klimov]

Describe the bug

Even given a bulk of queries, RedisConnection#WriteItem() loops them calling WriteOne().
The latter calls WriteRESP() and async_flush().

Expected behavior

Write everything queued, flush once.

Your Environment

c5c7925

Additional context

While more async I/O may even reduce mutex contention, this is not what our buffered streams are for.