It seems that current approach stops workring after amount of parallel
connection exceeds ~500. This PR introduces an 'async' feature which
enable asyncronius, tokio based implementation of stratum server.
* fix bug with passing current_difficulty in Handler
I passed in Handler current_difficulty as an u64, so it was copied
and all later changes were ignored. In this commit I pass referece on
RwLock with current_difficulty.
* fix crash in build_block
* improve error handling
* print error message on unknown error
* replace original server by tokio based one
* fixes after review
* reduce sleep time in mail loop to 5ms
e-max
0d36acf01b