Implement transaction pool sync

[billfort]

Proposed changes in this pull request

this new feature is related to issue: #832

• define new message types of SYNC_TXN_... in nodemessage.proto
• create a new file synctxnpool.go in lnode package, to process sync txn pool functions.
• add message handler and start go ruotine of syncing txn pool in localNode.initSyncing method.
• sync txn pool do as following:
  • each node sycing txn at a rand duration about 10 seconds
  • node send to some neighbors ask for their txn pool's hash
  • node compare its own txn pool hash with recevied hash, and compare their txn count too.
  • if hash is different, node choose a neighbor has biggest txn count to request sync txns.
  • after get txns, the node update its txn pool

Type (put an x where ever applicable)

• [] Bug fix: Link to the issue
• Feature (Non-breaking change)
• Feature (Breaking change)
• Documentation Improvement

Checklist

Please put an x against the checkboxes. Write a small comment explaining if its N/A (not applicable)

• Read the CONTRIBUTION guidelines.
• All the tests are passing after the introduction of new changes.
• Added tests respective to the part of code I have written.
• Added proper documentation where ever applicable (in code and README.md).
• Code has been written according to NKN-Golang-Style-Guide

Extra information

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