SECURITY: Fix potential denial of service attack against PostSRSd

I discovered that PostSRSd could be tricked into consuming a lot of CPU
time with an SRS address that has an excessively long time stamp tag,
e.g.

SRS0=HHHH=TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT=0@example.com