KUDU-3393  C++ client support split a tablet to mutil ranges and concurrent scan data

I add a param for build 'KuduScanToken', like this :
`
KuduScanTokenBuilder builder(table);
vector<KuduScanToken*> tokens;
ElementDeleter deleter(&tokens);
// set splitSizeBytes
builder.SetSplitSizeBytes(1000);
ASSERT_OK(builder.Build(&tokens));
`
The default value of split_size_bytes is 0, and this means we don't split the key range
for a tablet.
If the value of split_size_bytes is nonzero, we will try to send a SplitKeyRangeRPC to
tservers. We may get more tokens than tablets num and the more tokens will help us scan faster.

Change-Id: I207f9584cd558d32fcd9e8de7d6c25e517377272
Reviewed-on: http://gerrit.cloudera.org:8080/18945
Tested-by: Kudu Jenkins
Reviewed-by: Yingchun Lai <acelyc1112009@gmail.com>
7 files changed