Tweak CachePolicy construction

[CosmicHorrorDev]

Changes the constructors to return Result<CachePolicy, NotStorable> where NotStorable is a wrapper that indicates the inner CachePolicy MUST NOT be stored, along with changing the base constructor to take an explicit time instead of internally calling SystemTime::now(). Both of these changes were handled through adding new constructors and deprecating the old ones

old

impl CachePolicy {
    pub fn new(&Req, &Res) -> Self { ... }
    pub fn new_options(&Req, &Res, SystemTime, CacheOptions) -> Self { ... }
}

new

impl CachePolicy {
    pub fn try_new(&Req, &Res, SystemTime) -> Result<Self, NotStorable> { ... }
    pub fn try_new_with_options(&Req, &Res, SystemTime, CacheOptions) -> Result<Self, NotStorable> { ... }
}

closes #13 closes #14