add meter implementation

When packet drop happens, new error code ERROR_DROP_PACKET will
be returned to pipeline.
fix meter burst bucket drain speed (kilobits)
prevent wrap-around for rate limiter
fix gcc warnings with -Wall -Wextra
nsec was 10^-9 sec
DSCP calculation

Handles AF classes only 1-4 encoded in first 3 bits (RFC2597).
The final bit in DSCP must be 0 by RFC2474.
The addition performs in 1-3 in 4-6 bits, excluding 000.
fix value size to be that of specification
reduce log output, which is normal in operation
add flow removal triggered by meter deletetion

removal of a meter will also remove associated flows by specification.
performance tuning
Merge pull request #120 from iHiroakiKawai/meter

add meter implementation