fix: remove read barrier from postjoin

When instance runs `postjoin` function, it's supposed to have both
target and current grade Offline. Taking a read barrier in that state
makes no sence. Instead, an instance should set target grade online
asap. Ensuring it syncs is the responsibility of the governor.

Excessive read barrier affects `restart_both` test. One of the instances
(i1 who becomes a follower) sends a `read_index` request to i2 (leader).
But i2 reveals  `self.commit_to_current_term()` condition isn't met and
drops the request. As a result, i1 is obliged to wait for 10s timeout
before retrying `read_index` request again, but the test timeouts even
sooner.