btcjson: Update fields in GetBlockChainInfoResult

Update the fields of GetBlockChainInfoResult to reflect the current state of
the RPC returned by other full-node implementations.

 * InitialBlockDownload - Node is in Initial Block Download mode if True.
 * SizeOnDisk - The estimated size of the block and undo files on disk.

btcjson/chainsvrresults.go

@@ -221,9 +221,11 @@ type GetBlockChainInfoResult struct {
 	Difficulty           float64 `json:"difficulty"`
 	MedianTime           int64   `json:"mediantime"`
 	VerificationProgress float64 `json:"verificationprogress,omitempty"`
+	InitialBlockDownload bool    `json:"initialblockdownload,omitempty"`
 	Pruned               bool    `json:"pruned"`
 	PruneHeight          int32   `json:"pruneheight,omitempty"`
 	ChainWork            string  `json:"chainwork,omitempty"`
+	SizeOnDisk           int64   `json:"size_on_disk,omitempty"`
 	*SoftForks
 	*UnifiedSoftForks
 }