chain: Request block ntfns only after initial sync

[matheusd]

Previously, request for notifications about connected blocks were sent to the underlying dcrd instance before the chain was fully synced. This could cause a race issue if the underlying dcrd instance was also performing an initial sync, causing blocks to be connected by the blockConnected handler before the active data filter was loaded, causing transactions to be missed and the wallet's balance to be wrong, requiring a rescan to be fixed.

This could happen, for example, if both the dcrd and dcrwallet processes were started at the same time or if dcrd took just enough time to connect to the network that the wallet was also initialized (for exemple, from within Decrediton).

This fixes the issue by refactoring the initial sync code to only request block notifications after the initial header sync has been completed by the wallet.

Note that, despite a large(ish) diff, this is mostly a code move PR.

One way to reproduce the above issue in simnet is the following:

• Run dcrd's tmux script to setup a full simnet env
• Create a new, empty wallet (wallet3)
• Run, complete the initial sync and generate an address for wallet3
• Shutdown the wallet
• Send (but do not mine) 1 DCR from wallet1 to the previously generated wallet3 address
• Apply the diff below to the current master, to increase the odds of the issue happening
• Run wallet3 with the diff applied
• Before the 5 second timeout added by the diff elapses, mine a block that includes the transaction generated above
• The wallet will not see the transaction and will remain with a zero balance

diff --git a/chain/sync.go b/chain/sync.go
index 0881e218..583cdcb2 100644
--- a/chain/sync.go
+++ b/chain/sync.go
@@ -13,6 +13,7 @@ import (
    "runtime/trace"
    "sync"
    "sync/atomic"
+   "time"

    "decred.org/dcrwallet/v4/errors"
    "decred.org/dcrwallet/v4/rpc/client/dcrd"
@@ -328,6 +329,12 @@ func (s *Syncer) Run(ctx context.Context) (err error) {
        return err
    }

+   log.Infof("XXXXXXXXXXXXXXX Gonna wait 5 seconds")
+   for i := 0; i < 5; i++ {
+       time.Sleep(time.Second)
+       log.Infof("XXXXXXXXXXXX %d", i+1)
+   }
+
    cnet := s.wallet.ChainParams().Net
    s.fetchHeadersStart()
    for {

[jrick]

needs a rebase