Add PoW consensus bundle as a module within BLEEP library

[ygnkim]

To give a PoW bundle for the users of BLEEP, it is required to implement PoW module within a BLEEP library.

Tentatively, following tasks are required for adding a PoW module.

• Add ProtocolLayer implementation for PoW.
• Add Tx generator and TxPool within ProtocolLayer implementation (of PoW).
• Add PoW block data structure and mining function with tentative block pool data strucrue.
• Add Tx broadcasting, Block broadcasting mechanism (gossip protocol)
• Add blockchain data structure and consensus (ordering) protocol.

[ygnkim]

PoW is successfully implemented as a module within BLEEPlib. Appropriate modules (classes) are added within BL3_protocol directory. In other words, PoW is implemented as a protocol in BL3 layer (operating upon a Peer(BL2) layer).

First of all, basic block data structure is implemented (called POWBlock). Then, in order to share the POWBlock among different nodes, and to synchronize the ordering of the blocks among nodes, additional data structure which represents full snapshot of the blocks is implemented. (called BlockTree) Then, the propagation&synchronization protocol is implemented. (using protocol messages such as POWBlockGossipInventory, POWBlockGossipGetBlocks, POWBlockGossipGetData, POWBlockGossipBlk)

I have confirmed that PoW blocks are synchronized successfully among 10 different nodes (see BLEEPlib-pow-treesync-2 in regtest/3_BLEEPlib/3_PoW_treesync).

[ygnkim]

Merged to develop branch by PR #324