Replace message type literals with protocol.h constants
Replace message type literals with protocol.h constants

Summary:
No reason to use literals when there exists constant variables that convey the
same information.  This reduces the number of places that need to be updated if
a protocol change occurs and makes the code more consistent with the node
software.

sed -i s/\"version\"/NetMsgType::VERSION/g
sed -i s/\"verack\"/NetMsgType::VERACK/g
sed -i s/\"getaddr\"/NetMsgType::GETADDR/g
sed -i s/\"addr\"/NetMsgType::ADDR/g

Test Plan:
  ninja
  ninja check-bitcoin-seeder
  cd src/seeder
  grep -rnI \"version\"
  grep -rnI \"verack\"
  grep -rnI \"getaddr\"
  grep -rnI \"addr\"
`grep`ing for the literals should return nothing in the seeder directory

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6182

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