worker: remove data worker

It will require rewriting to work in the new framework. This code can
still be found in the history, but until then it's pointless to keep
around.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
cli: fix type of number of workers option

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
md partial sync: automatically renew log lock

Run a separate thread to renew the lock every timeout / 2 period to
guard against lock expiration.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>