Preinitialize entire channel map before using it in parallel restore.

Trying to assign channels to the map, and spin up a go routine that uses
the map in the same loop can result in the following panic at run time.

`fatal error: concurrent map read and map write`

This can also be detected using golang's race condition detector

```
go run --race gprestore.go --timestamp 20240201180314 --create-db
--redirect-db test2 --jobs 8 --verbose
...
==================
WARNING: DATA RACE
Read at 0x00c0003a6300 by goroutine 77:
  runtime.mapaccess1_fast64()
      /usr/local/go/src/runtime/map_fast64.go:13 +0x0
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements.func1()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:158 +0x1c4
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements.func2()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:159 +0x41

Previous write at 0x00c0003a6300 by main goroutine:
  runtime.mapassign_fast64()
      /usr/local/go/src/runtime/map_fast64.go:93 +0x0
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:144 +0x805
  github.com/greenplum-db/gpbackup/restore.ExecuteRestoreMetadataStatements()
      /home/kyeap/workspace/gpbackup/restore/wrappers.go:321 +0xa4
  github.com/greenplum-db/gpbackup/restore.restorePredata()
      /home/kyeap/workspace/gpbackup/restore/restore.go:334 +0x9d1
  github.com/greenplum-db/gpbackup/restore.DoRestore()
      /home/kyeap/workspace/gpbackup/restore/restore.go:163 +0x184
 ```

restore/parallel.go

@@ -139,12 +139,15 @@ func ExecuteStatements(statements []toc.StatementWithType, progressBar utils.Pro
 		panicChan := make(chan error)
 		splitStatements := scheduleStatementsOnWorkers(statements, connectionPool.NumConns)
 		chanMap := make(map[int]chan toc.StatementWithType, connectionPool.NumConns)
+		// preinitialize channels to prevent concurrent read and write
 		for i := 0; i < connectionPool.NumConns; i++ {
 			chanMap[i] = make(chan toc.StatementWithType, len(splitStatements[i]))
 			for _, statement := range splitStatements[i] {
 				chanMap[i] <- statement
 			}
 			close(chanMap[i])
+		}
+		for i := 0; i < connectionPool.NumConns; i++ {
 			workerPool.Add(1)
 			go func(connNum int) {
 				defer func() {