Fix ketama quorum (thanos-io#5910)

* Fix quorum calculation for Ketama hashring

The quorum calculation is currently broken when using the Ketama
hashring. The reasons are explained in detail in issue
thanos-io#5784.

This commit fixes quorum calculation by tracking successfull writes
for each individual time-series inside a remote-write request.

The commit also removes the replicate() method inside the Handler
and moves the entire logic of fanning out and calculating success
into the fanoutForward() method.

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

* Fix error propagation

Signed-off-by: fpetkovski <filip.petkovsky@gmail.com>

* Fix writer errors

Signed-off-by: fpetkovski <filip.petkovsky@gmail.com>

* Separate write from replication errors

Signed-off-by: fpetkovski <filip.petkovsky@gmail.com>

* Add back replication metric

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

* Address PR comments

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

* Address code review comments

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>
Signed-off-by: fpetkovski <filip.petkovsky@gmail.com>