refactor: wantlist splits into WL and ThreadSafe WL

bitswap keeps the threadsafe version. observing the ledger shows that it
doesn't need it anymore (ledgermanager is protected and safe).

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>
rename to strategy.LedgerManager to decision.Engine

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>
rm empty file

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>