Update benchmark_test.c for new function interface

I have try to compile benchmark_test.c and found it was a little out of date so that cannot be compiled.
Then I make some modification on it and it work now.

When compile with gcc (V4.6.3), there were 3 warnings:

> benchmark_test.c: In function ‘batch_insert_small_test’:
> benchmark_test.c:189:9: warning: passing argument 3 of ‘mongo_insert_batch’ from incompatible pointer type [enabled by default]
> /home/newk/repo/mongo-c-driver/src/mongo.h:471:18: note: expected ‘const struct bson **’ but argument is of type ‘struct bson **’
> benchmark_test.c: In function ‘batch_insert_medium_test’:
> benchmark_test.c:207:9: warning: passing argument 3 of ‘mongo_insert_batch’ from incompatible pointer type [enabled by default]
> /home/newk/repo/mongo-c-driver/src/mongo.h:471:18: note: expected ‘const struct bson **’ but argument is of type ‘struct bson **’
> benchmark_test.c: In function ‘batch_insert_large_test’:
> benchmark_test.c:225:9: warning: passing argument 3 of ‘mongo_insert_batch’ from incompatible pointer type [enabled by default]
> /home/newk/repo/mongo-c-driver/src/mongo.h:471:18: note: expected ‘const struct bson **’ but argument is of type ‘struct bson **’

I have no idea how to deal with them, any suggestion?