New workload for delete rows/ranges

[juliayakovlev]

We need to be able to test the workloads:

1. Writes:

mix of :
- row writes 
- row overwrites 
- row deletes
- partition range deletes.

2. Reads:

mix of: 
- row reads 
- partition scans
- range scans

Seems with current state of scylla-bench we can perform most of these actions except of rows/range deletion. So we need to add to scylla-bench a new workload delete. It should be a few modes:

• row deletion
• partition range deletions
• range deletions (new feature https://github.com/scylladb/scylla/issues/432)

Also we should be able to validate the deletion's result.

[juliayakovlev]

@roydahan