block_timestamp::maximum returns a small value

[tbfleming]

block_timestamp::maximum has this incorrect definition:

static block_timestamp maximum() { return block_timestamp( 0xffff ); }

I recommend the following:

• Remove this function
• Create a replacement with a different name (e.g. max()) to prevent existing contracts that rely on maximum from changing behavior. Contracts which rely on the incorrect behavior will fail to build, notifying the authors of the potentially serious problem.