tools/stellar-archivist: Fix stellar-archivist dumpxdr command

[bartekn]

DumpXdrAsJson function is creating shared objects for various XDR types:

var lhe xdr.LedgerHeaderHistoryEntry
var the xdr.TransactionHistoryEntry
var thre xdr.TransactionHistoryResultEntry
var bke xdr.BucketEntry
var scp xdr.ScpHistoryEntry

These objects are later reused inside for loop. It seems that previous values are persisted in the shared objects when unmarshaling unions. For example:

stellar-archivist dumpxdr bucket-4100ad3b1085bd14d1c808ece3b38db97171532d0d11ed5edd57aff0e416e06a.xdr.gz

56052390-}{
56052391-    "Type": 1,
56052392-    "LiveEntry": {
56052393-        "LastModifiedLedgerSeq": 23608331,
56052394-        "Data": {
56052395-            "Type": 0,
56052396-            "Account": {
56052397:                "AccountId": "GDZZYKUL3B5R2CNZF6GWZGQB6W4DAYMUBU4H7YHSTY5K4VJYEO7N2X73",
56052398-                "Balance": 15998000,
56052399-                "SeqNum": 92133538194784276,
56052400-                "NumSubEntries": 1,
56052401-                "InflationDest": null,
56052402-                "Flags": 0,
56052403-                "HomeDomain": "",
56052404-                "Thresholds": [
56052405-                    1,
56052406-                    0,
56052407-                    0,
56052408-                    0
56052409-                ],
56052410-                "Signers": null,
56052411-                "Ext": {
56052412-                    "V": 0,
56052413-                    "V1": null
56052414-                }
56052415-            },
56052416-            "TrustLine": null,
56052417-            "Offer": null,
56052418-            "Data": null
56052419-        },
56052420-        "Ext": {
56052421-            "V": 0
56052422-        }
56052423-    },
56052424-    "DeadEntry": {
56052425-        "Type": 0,
56052426-        "Account": {
56052427-            "AccountId": "GDZZYMG4436ILOHHRZKWFJV3SRRRLH55ZRXCD5CXYL2XSAW3TYHKJU4I"
56052428-        },
56052429-        "TrustLine": null,
56052430-        "Offer": null,
56052431-        "Data": null
56052432-    },
56052433-    "MetaEntry": null
56052434-}{

Note: AccountId fields are strkey-encoded for readability. In example above LiveEntry should be null because bucket entry type is equal 1.

This PR recreates empty objects on each iteration.

---

Should a long term fix be actually added to go-xdr (xdr.Unmarshal)?

[bartekn]

It looks like I accidentally included a fix for this in cca9f0a#diff-a180e1f12313289bc086aad14e4310c5 (PR: #1264). Closing this but the question remains:

Should a long term fix be actually added to go-xdr (xdr.Unmarshal)?