init: don't start indexes sync thread prematurely

By moving the 'StartIndexes()' call into the 'initload'
thread, we can remove the threads active wait. Optimizing
the available resources.

The only difference with the current state is that now the
indexes threads will only be started when they can process
work and not before it.

src/index/base.cpp

@@ -163,12 +163,6 @@ static const CBlockIndex* NextSyncBlock(const CBlockIndex* pindex_prev, CChain&
 
 void BaseIndex::ThreadSync()
 {
-    // Wait for a possible reindex-chainstate to finish until continuing
-    // with the index sync
-    while (!g_indexes_ready_to_sync) {
-        if (!m_interrupt.sleep_for(std::chrono::milliseconds(500))) return;
-    }
-
     const CBlockIndex* pindex = m_best_block_index.load();
     if (!m_synced) {
         std::chrono::steady_clock::time_point last_log_time{0s};