compaction: scrub/validate: prevent printing non-utf8 partition keys
compaction: scrub/validate: prevent printing non-utf8 partition keys

Corrupt keys might be printed as non-utf8 strings to the log,
and that, in turn, may break applications reading the logs,
such as Python (3.7)

For example:
```
Traceback (most recent call last):
  File "/home/bhalevy/dev/scylla-dtest/dtest.py", line 1148, in tearDown
    self.cleanUpCluster()
  File "/home/bhalevy/dev/scylla-dtest/dtest.py", line 1184, in cleanUpCluster
    matches = node.grep_log(expr)
  File "/home/bhalevy/dev/scylla-ccm/ccmlib/node.py", line 367, in grep_log
    for line in f:
  File "/usr/lib64/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb3 in position 5577: invalid start byte
```

Test: unit(dev)
DTest: scrub_with_one_node_expect_data_loss_test

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20210730105428.2844668-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla