The typedef int64_t NodeId is used in several places which generally have no need for anything else in net.h.

Specifically the introduction of net/util.h prevents a circular dependency in the next commit.

Moving MAX_PROTOCOL_MESSAGE_LENGTH out of net.h is needed to prevent a circular dependency in the upcoming commit that moves Transport to Net.

This commit also adds seperate header and implementation files
for TransportV1 and TransportV2.

This removes the Transport dependency on chainparams.h (libbitcoin_common).

libbitcoin_wallet has no dependency on libbitcoin_net.

Drop the unused include so the next commit can move
protocol.h to libbitcoin_net without adding such a
dependency.

GenerateRandomKey() is used by V2Transport to generate the ephemeral keys used in the handshake.

The ci "test-each-commit" job fetches the PR branch being tested with a depth of (# of commits in PR + 2), and then tries to run tests on commits after the most recent merge commit.

When a PR is opened against a bitcoin core branch, a merge commit is always guaranteed to exist within the fetch depth, because bitcoin core branches always point at merge commits.

However, in fork repositories, pull requests can be opened based on other branches that don't contain recent merge commits, and this will currently cause git rev-list to fail with fatal: bad revision '^^@'.

Work around this problem by not requiring a recent merge commit, and just testing on all fetched commits if a merge commit can't be found.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>