This commit lets etcdserver skip needless log entry applying. If the
result of log applying isn't required by the node (client that issued
the request isn't talking with the node) and the operation has no side
effects, applying can be skipped.
It would contribute to reduce disk I/O on followers and be useful for
a cluster that processes much serializable get.
Provides two implementations of Recorder-- one that is non-blocking
like the original version and one that provides a blocking channel
to avoid busy waiting or racing in tests when no other synchronization
is available.
lorneli