Improve chain state init efficiency

[lindlof]

Remove unnecessary slice of all block indexes and
remove DB iteration over all block indexes that
used to determined the size of the slice

[lindlof]

Build and Test / Go CI (1.14) test failed for 624cbf3. Re-ran tests by pushing again with a different commit message and they succeeded.

Seem to be a fluke in address management and most certainly has nothing to do with this PR.

Run sh ./goclean.sh
+ go test -tags=rpctest ./...
ok  	github.com/btcsuite/btcd	0.035s
--- FAIL: TestAddrManagerSerialization (0.00s)
    addrmanager_internal_test.go:108: expected to find 5 addresses, found 4
FAIL
FAIL	github.com/btcsuite/btcd/addrmgr	0.148s
ok  	github.com/btcsuite/btcd/blockchain	2.676s
?   	github.com/btcsuite/btcd/blockchain/fullblocktests	[no test files]
ok  	github.com/btcsuite/btcd/blockchain/indexers	0.035s
ok  	github.com/btcsuite/btcd/btcec	1.686s
ok  	github.com/btcsuite/btcd/btcjson	0.007s
ok  	github.com/btcsuite/btcd/chaincfg	0.006s
ok  	github.com/btcsuite/btcd/chaincfg/chainhash	0.006s
?   	github.com/btcsuite/btcd/cmd/addblock	[no test files]
?   	github.com/btcsuite/btcd/cmd/btcctl	[no test files]
?   	github.com/btcsuite/btcd/cmd/findcheckpoint	[no test files]
?   	github.com/btcsuite/btcd/cmd/gencerts	[no test files]
ok  	github.com/btcsuite/btcd/connmgr	0.131s
ok  	github.com/btcsuite/btcd/database	0.033s
?   	github.com/btcsuite/btcd/database/cmd/dbtool	[no test files]
ok  	github.com/btcsuite/btcd/database/ffldb	5.439s
ok  	github.com/btcsuite/btcd/database/internal/treap	0.021s
ok  	github.com/btcsuite/btcd/integration	7.677s
ok  	github.com/btcsuite/btcd/integration/rpctest	3.245s
?   	github.com/btcsuite/btcd/limits	[no test files]
ok  	github.com/btcsuite/btcd/mempool	0.236s
ok  	github.com/btcsuite/btcd/mining	0.031s

[Rjected]

Tested, LGTM unless others have objections to doing it this way

[jcvernaleo]

OK