v0.2.0

Changes:

• Introducing the expire attribute to specify challenge expiration. Upon expiration, the state is automatically updated to expired.
• Implemented multithreading support and introduced a new attribute workers to specify the number of concurrent workers.

Migrating from 0.1.x:

• Multithreading is now enabled by default, resulting in significantly faster PoW (approximately 6-10x faster). If migrating from version 0.1.x, it is advisable to consider increasing the complexity (the maximum random number on the server).
• The default value of maxnumber is now 1,000,000 (instead of 10 million).