Add the epoch-sleep helper, which queries the current epoch and then
sleeps until the epoch is greater (polling once a second). This can
replace the multiple namadac invocations used in e2e tests.

Instead of manually querying, we now epoch-sleep until the next epoch,
limiting the number of clients invoked to 1.

Also update the epoch sleep e2e test to check the return value (parsed
from the output of epoch-sleep).

PoS hammered the node as fast as possible with epoch queries in its
tests to wait for a specified epoch. Instead, epoch-sleep repeatedly
until the target epoch has been reached.

This could be improved further by adding an --until parameter to
epoch-sleep.