BUG-529: improve droptest performance

This eliminates the use of synmchronized blocks to perform statistics
maintenance.

(cherry picked from commit e339e96270ade70943c53b8af2d6a61677ac0686)
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Change-Id: I731ee2f045ffaf5b03de3a0ac8ee927a3baa0d2e