#1955 - Make FileSpout work in distributed mode

[dpol1]

FileSpout read the seed directory and filled its input queue inside the constructor. In distributed mode the constructor runs only on the client that submits the topology, so the resolved queue (usually empty, since the seed files live on the workers) was serialised and shipped out. The spout then sat idle with no activity in the logs.

The constructors now only keep the values they are given: the directory and filter, or the explicit file list. The seeds are resolved in open(), which runs on each worker. WARCSpout already calls super.open(), so it gets the same behaviour without any change.

While moving the code I also fixed a log call that used printf-style %s formatting instead of SLF4J {} placeholders.

Tests: a regression test builds the spout against an empty directory and creates the seed file only after construction, before open(). It fails on the old code and passes now. There is also a directory happy-path test and a serialisation round-trip.

Closes #1955

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[jnioche]

looks good, a few minor suggestions

[jnioche]

Thanks @dpol1