[tools] introduced '--seq_start' flag
[tools] introduced '--seq_start' flag

Introduced '--seq_start' flag for 'kudu test loadgen' tool.
Use this new flag to specify initial value for generator in sequential
mode.  The new flag facilitates running the tool against already
existing table multiple times.

Here is an example: an already existing table t3 has 5 columns.
The sequence of 3 runs below inserts 6000 rows in total
(3 runs * 1000 rows per thread * 2 threads)
with no duplicate values across all columns:

  kudu test loadgen 127.0.0.1 --table_name=t3 --num_threads=2 \
    --num_rows_per_thread=1000 --seq_start=0

  kudu test loadgen 127.0.0.1 --table_name=t3 --num_threads=2 \
    --num_rows_per_thread=1000 --seq_start=10000

  kudu test loadgen 127.0.0.1 --table_name=t3 --num_threads=2 \
    --num_rows_per_thread=1000 --seq_start=20000

Also, introduced '--table_num_replicas' flag
to specify number of replicas for auto-created tables.

Renamed '--num_buckets' into '--table_num_buckets'.

Besides, do not abort loadgen tool if generator thread encounters
an error.  Instead, stop generating data and gracefully join.

Change-Id: I0899168b31d4e3a2d53fff7db669a0840452f557
Reviewed-on: http://gerrit.cloudera.org:8080/4672
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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