api: add stratum difficulty

[tdb3]

Fixes Issue #482
Adds current stratum difficulty to the api/system/info endpoint.

Tested mining on testnet4 (difficulty 1000)

$ curl <bitaxe_IP_address>/api/system/info
{
...
        "stratumDiff":  1000,

then stopped the testnet4 stratum server to induce failover to the secondary stratum server (mainnet, Ocean):

$ curl <bitaxe_IP_address>/api/system/info
{
...
        "stratumDiff":  262144,

[skot]

This works great, thanks @tdb3 !