QPID-4748: Consistent handling of durations in broker configuration, allowing sub-second intervals.

Provides string conversion for sys::Duration, allowing intervals to be expressed like this:

10.5 - value in seconds, backward compatible.
10.5s - value in seconds
10.5ms - value in milliseconds
10.5us - value in microseconds
10.5ns - value in nanoseconds

Converted the folllowing broker options to Duration:
mgmtPubInterval, queueCleanInterval, linkMaintenanceInterval, linkHeartbeatInterval

Did not convert: maxNegotiateTime. This is expressed in milliseconds so it would not be
backward compatible to make it a Duration.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1469661 13f79535-47bb-0310-9956-ffa450edef68