fsmonitor--daemon: create token-based changed path cache

Teach fsmonitor--daemon to build a list of changed paths and associate
them with a token-id.  This will be used by the platform-specific
backends to accumulate changed paths in response to filesystem events.

The platform-specific file system listener thread receives file system
events containing one or more changed pathnames (with whatever
bucketing or grouping that is convenient for the file system).  These
paths are accumulated (without locking) by the file system layer into
a `fsmonitor_batch`.

When the file system layer has drained the kernel event queue, it will
"publish" them to our token queue and make them visible to concurrent
client worker threads.  The token layer is free to combine and/or de-dup
paths within these batches for efficient presentation to clients.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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