Better memory management

[pszturmaj]

See https://github.com/pszturmaj/ddb/blob/f45f0ba7961929254e6a63b346b20199e802f422/source/ddb/postgres.d#L1049

This was supposed to be temporary, until I'd finish other parts, but stayed here until now. It needs to be rewritten to reuse the buffer if possible. I would leave it as is for big messages, with configurable threshold in bytes and for messages smaller than threshold I would reuse a growable buffer. This change should prevent many unnecessary allocations and is fairly simple to do.

A little more complex but better solution would be receiving a fixed size buffer, then consuming that buffer field by field, and when you read past end of the buffer it should refresh the buffer with new content under the hood.