Add reconfig poll patch

[gkumbhat]

This patch is essentially porting the patch submitted by @njhill in PR #1518 to 2.4.0 version

Currently when the configured model list is updated via a call to handleReloadConfigRequest, the request thread blocks until any newly added models become available.

Their availability however depends on the filesystem polling thread rescanning the filesystem at some periodic interval, meaning that there's an arbitrary delay before the requested changes actually take effect and the RPC returns.

This problem may not be very noticeable with the default polling interval of 1 second, but seems undesirable for longer intervals and in particular makes API-based dynamic reconfiguration incompatible with the --file_system_poll_wait_seconds=0 setting (in this case all handleReloadConfigRequest calls time-out and do not take effect).

Fixes #1519

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