handle rate limiting

[briskt]

Most APIs have rate limiting to prevent excess usage, which causes failure if personnel-sync has many records to update at once. The REST API adapter should implement some type of retry to handle this gracefully.

[briskt]

The config options BatchSize and BatchDelaySeconds are a crude implementation that works most of the time. They aren't well documented, and may not work in every situation. For instance, do the "list" requests count against rate limit counters? If so, since the number of "list" requests are needed is unknown at the time the configuration is written, it is not possible to know how large to set the BatchSize parameter.