Was incorrectly trimming the trailing '.' from the target; this in turn
caused the etcd server to accept any SRV record with an IP target
instead of only targets with A records.

Whenever the WAL is opened for writes, it should write zeroes to its tail
starting from the first zero record. Otherwise, if there are entries past
the first zero record due to a torn write, any new writes that overlap the
old entries will lead to a garbage record on the tail and cause a CRC
mismatch.

If a user upgrades etcd from 2.3.x to 3.0 and shutdown the
cluster immediately without triggering any new backend writes,
then the consistent index in backend would be zero.

The user cannot restart etcdserver due to today's strick index
match checking. We now have to lose this a bit for this case.

A buffered writer that only writes full pages or when explicitly flushed.

Forces torn writes to only happen on sector boundaries.

Fixes #6271

Windows requires this lock to be released before the directory is
renamed. But on unix-like operating systems, releasing the lock and
trying to reacquire it immediately can be flaky if a process is forked
around the same time. The file descriptors are marked as close-on-exec
by the Go runtime, but there is a window between the fork and exec where
another process will be holding the lock.

After winning an election or obtaining a lock, we
auto-append a slash after the provided key prefix.
This avoids the previous deadlock due to waiting
on the wrong key.

Fixes #6278

Conflicts:
	clientv3/concurrency/election.go
	clientv3/concurrency/mutex.go