Add rebalance functionality in ZMT

[ganning127]

features

• start
• stop; remove timestamp from resource in the middle of rebalance (/metadata)
• progress
• only show for replication resources, possibly a third tab
• only if there's a replication resource

command line rebalance
iadmin modresc replResc rebalance

problems

• cpp API will block a second request if the first one is already running
• rebalance doesn't return anything until its complete
• don't know how the rebalance operation is going to take (no dry run)

[korydraughn]

When a rebalance is started, metadata is attached to resource. The metadata acts as a lock which blocks new rebalance requests on the target resource.

On completion, the metadata is removed.

See https://docs.irods.org/4.3.0/plugins/composable_resources/#rebalancing.