Add BufferedMessageEncoder, fix PSQLFrontendMessageEncoder

[fabianfett]

Motivation

• We want to communicate as efficiently as possible to the server. For this reason we use message pipelining within a query.
• If we drop the current use of MessageToByteHandler and do the encoding in the PSQLChannelHandler, we can write to a single ByteBuffer and only forward this together with a flush

Changes

• Add BufferedMessageEncoder
• Rename PSQLFrontendMessage.Encoder to PSQLFrontendMessageEncoder (got its own file)

[gwynne]

LGTM