Switch to use_existing_db=1 for updaterandom and mergerandom

Summary:
Without this change about half of the updaterandom reads and merge puts will be for keys that don't exist.
I think it is better for these tests to start with a full database and use fillseq to fill it.

Task ID: #

Blame Rev:

Test Plan:
Revert Plan:

Database Impact:

Memcache Impact:

Other Notes:

EImportant:

- begin *PUBLIC* platform impact section -
Bugzilla: #
- end platform impact -

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D35043

tools/benchmark.sh

@@ -198,7 +198,7 @@ function run_rangescanwhilewriting {
 function run_updaterandom {
   echo "Read/Modify/Write $num_keys random keys (not using merge).."
   cmd="./db_bench $params_w --benchmarks=updaterandom \
-       --use_existing_db=0 \
+       --use_existing_db=1 \
        --num=$num_keys \
        --sync=$syncval \
        --disable_data_sync=0 \
@@ -210,7 +210,7 @@ function run_updaterandom {
 function run_mergerandom {
   echo "Read/Modify/Write $num_keys random keys (using merge operator).."
   cmd="./db_bench $params_w --benchmarks=mergerandom \
-       --use_existing_db=0 \
+       --use_existing_db=1 \
        --num=$num_keys \
        --sync=$syncval \
        --disable_data_sync=0 \