Duplicate layer of AppendBatchSizeTrackers

[RaulGracia]

Problem description
The writer uses AppendBatchSizeTracker objects to infer the right size of batches to be written to the server. However, currently there are 2 places in which such AppendBatchSizeTracker objects are being instantiated: FlowHandler and TcpClientConnection. In principle, the batching logic should be only in one place, and concretely, at the connection level. This is because, without connection pooling enabled, he number of flows should be equal to the number of connections. With connection pooling enabled, is at the connection level at where we need to track the data being written to the server, which can be a mix of multiple multiplexed flows.

In fact, after a preliminary analysis, it looks like the AppendBatchSizeTracker objects instantiated in FlowHandler are not used at all, so we can safely remove them.

Problem location
Unnecessary AppendBatchSizeTracker objects instantiated in FlowHandler.

Suggestions for an improvement
Remove unnecessary code.