Make min difficulty algorithmic

[HardhatChad]

• If the min difficulty is constant, we potentially run into an issue longterm where base reward rate can't go lower (at 1 atomic unit) but global hashpower is large enough to exceed the 1 ore/min supply function.
• This pr sets a valid range for the base reward rate. If the base reward rate breaks this range, we increment or decrement the min difficulty accordingly.

[miralandlabs]

According to your latest source code below, INITIAL_BASE_REWARD_RATE is too small, which means in on-boarding period, the ORE supply is far less than 1 ORE/min (which requires millions of miners on average from initial launch) Also, BASE_REWARD_RATE_MAX_THRESHOLD (256) is too small, maybe 2u64.pow(16) or 2u64.pow(24) is an appropriate choice.

/// The base reward rate to intialize the program with.
pub const INITIAL_BASE_REWARD_RATE: u64 = 2u64.pow(6);

/// The minimum allowed base reward rate, at which point the min difficulty should be increased
pub const BASE_REWARD_RATE_MIN_THRESHOLD: u64 = 2u64.pow(5);

/// The maximum allowed base reward rate, at which point the min difficulty should be decreased.
pub const BASE_REWARD_RATE_MAX_THRESHOLD: u64 = 2u64.pow(8);