Introduce constant default variables to initialize seeder options
Introduce constant default variables to initialize seeder options

Summary:
Introduces constant default variables for the seeder's parameters.
This will minimize the number of changes necessary if the default
parameters are ever in need of being changed and also paves the way
to replace `CDnsSeedOpts` with gArgs in a future diff.

Test Plan:
  make
  ./src/bitcoin-seeder
This should output something like below:
  Supporting whitelisted filters: 0x1,0x5,0x11,0x15
  No nameserver set. Not starting DNS server.
  Loading dnsseed.dat...done
  Starting seeder...done
  Starting 96 crawler threads...2019-10-14T22:11:22Z connect() to [2600:8806:8600:1d0:a5f0:9863:3144:6f16]:28333 failed: Cannot assign requested address (99)
  ...
This checks `-w` defaults to filtering `0x1,0x5,0x11,0x15`, `-t` defaults to 96, `--wipeban`, `--wipeignore`, and `--testnet` default to `false`, and that `-o`, `-i`, `-k`, and `-ns` default to `nullptr`.

  ./src/bitcoin-seeder -n dev.bitframe.org
This should error out and print the following:
  Supporting whitelisted filters: 0x1,0x5,0x11,0x15
  No hostname set. Please use -h.
This checks that `-h` is defaulted to `nullptr`

  ./src/bitcoin-seeder -n dev.bitframe.org -h seeder.bitframe.org
This should error out and print the following:
  Supporting whitelisted filters: 0x1,0x5,0x11,0x15
  No e-mail address set. Please use -m.
This checks that `-m` is defaulted to `nullptr`.

  ./src/bitcoin-seeder -n dev.bitframe.org -h seeder.bitframe.org -m info@bitframe.org
This should output something like below:
  Supporting whitelisted filters: 0x1,0x5,0x11,0x15
  Loading dnsseed.dat...done
  Starting 4 DNS threads for seeder.bitframe.org on dev.bitframe.org (port 53).......done
  Starting seeder...done
  Starting 96 crawler threads...2019-10-14T23:51:43Z connect() to [2600:8806:8600:1d0:a5f0:9863:3144:6f16]:28333 failed: Cannot assign requested address (99)
  ...
This checks `-p` is defaulted to 53 and `-d` is defaulted to 4.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4250

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