Droptest: unify packet handling

Rework the two implementation to make sure they do equivalent
processing, such that the difference between the two really is only the
way they propage changes through the system.

Change-Id: I01cb2daea78747a5659d04da65c810188b3e38c6
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Droptest: optimize builder allocation

Builder can be reused between invocations, so let's cache it in a
thread-local variable.

Change-Id: Ib76f1d10aabd95d896d81c5be33d42230ee3f366
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Drop-test: remove minor janitorial things

- remove superfluous constructors
- remove supresswarnings
- optimize sb.append a tiny bit

Change-Id: Id9e4662f4212e0160bf4095f21a66d709f179372
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Use non-sign-extending shift

Just a precaution of intent: use shift which introduces 0, not
sign-dependent.

Change-Id: I370805c1d1dd5e9ba3893768767f1cb6047acbbd
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>