Support dynamic throttling of async persistence operations

Adds an operation throttler that is intended to provide global throttling
of async operations across all persistence stripe threads. A throttler
wraps a logical max pending window size of in-flight operations. Depending
on the throttler implementation, the window size may expand and shrink
dynamically. Exactly how and when this happens is unspecified.

Commit adds two throttler implementations:
 * An unlimited throttler that is no-op and never blocks.
 * A throttler built around the mbus `DynamicThrottlePolicy` and defers
   all window decisions to it.

Current config default is to use the unlimited throttler. Config changes
require a process restart.

Offers both polling and (timed, non-timed) blocking calls for acquiring
a throttle token. If the returned token is valid, the caller may proceed
to invoke the asynchronous operation.

The window slot taken up by a valid throttle token is implicitly freed up
when the token is destroyed.

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