ReplicationTasksStorage: Remove synchronized from list* methods

These synchronized statements were added to make the tests pass.
However, they have undesirable behavior when considering production
systems (such as the concern raised in [1]).

[1] https://gerrit-review.googlesource.com/c/plugins/replication/+/280357/comment/757c5227_0de2800d/

Change-Id: I5556ea318aed400eb43c60fdbda41106dd4a17ff