IMPALA-4829: Change default Kudu read behavior for "RYW"
IMPALA-4829: Change default Kudu read behavior for "RYW"

Currently the default Kudu read mode is set to "READ_LATEST",
which essentially provides no guarantees on reading except
that any read issued will read the latest value that the
target replica happens to have. This is not necessarily a
time after a previous write operation in the same session.
By changing the read mode to the misleadingly named
"READ_AT_SNAPSHOT", we can ensure that Kudu reads will all
be at times at least or greater than the latest "observed"
time (which Impala already sets on the client). Note that
this does not mean all reads are performed at the same
timestamp (i.e. a snapshot read) because that requires
setting a snapshot timestamp, but doing this will require
more work in the future in both Impala and (mostly) Kudu.
The Kudu team calls this "Read Your Writes".

This means that, after this change, values written within a
session will always be visible to subsequent reads. Before
this change, this was usually the case but not guaranteed.

Testing: Private test run, running an exhaustive job now.
This is otherwise difficult to validate in new tests. This
has plenty of time to bake for 2.9 in case we discover
performance or functional issues.

Change-Id: I4011f8277083982aee2c6c2bfca2f4ae2f8cb31e
Reviewed-on: http://gerrit.cloudera.org:8080/5802
Reviewed-by: Thomas Tauber-Marshall <55b5d0aded728f7a3075d2dcc6abf84f96c36406@cloudera.com>
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Impala Public Jenkins

michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala