[enh] Few cleaning (unused imports, tests which can randomly fail)

test/fast/ddos-test.js

test/fast/pow/pow-engine.js

test/integration/branches_revert_memberships.js

test/integration/identity-same-pubkey.js

test/integration/peerings.js

test/integration/server-sandbox.js

test/integration/v0.4-dividend.js

test/integration/v0.4-times.js

test/integration/v0.5-identity-blockstamp.js

test/integration/v0.5-transactions.js

test/integration/v0.6-difficulties.js

test/integration/v1.0-source-garbaging.js