Form security validation and purge performance

Rather than iterate through all form security tokens to validate and
purge tokens given to a page, partition the token data structure into
blocks by date, and use the token itself as the key. This allows for
constant-time lookups, as well as more performant purges of old token
data by purging entire days in one step.

Signed-off-by: David Hicks hickseydr@optusnet.com.au