Offload tracking current writer to `WriterFactory`

[ritvikos]

Overview

Currently, the Spooler is self-referential internally. The WriterFactory creates a new fileWriter, which is tracked by the currentWriter field in Spooler.

synapse/spooler/spooler.go

Lines 63 to 68 in f171ddd

	type Spooler struct {
	config SpoolerConfig
	writer writerFactory
	batcher *batcher
	currentWriter *fileWriter
	}

Scope

Offload the tracking part to WriterFactory itself and rename it to more relevant one, probably Writer.