dnsdist: add pooling support for RemoteLoggerInterface

dnsdist: add pooling support for RemoteLoggerInterface

This adds a new kind of `RemoteLoggerInterface`: `RemoteLoggerPool`.
It can take multiple other `RemoteLoggerInterface`s and pass data to
them in round-robin order by default.

This also adds additional option to `newRemoteLogger`, `newFrameStreamTcpLogger`
and `newFrameStreamUnixLogger`: `connectionCount`, which can be used to
generate a pool with multiple connections.

Closes: #14861