Merge #15323: rpc: Expose g_is_mempool_loaded via getmempoolinfo

effe81f750 Move g_is_mempool_loaded into CTxMemPool::m_is_loaded (Ben Woosley)
bb8ae2c419 rpc: Expose g_is_mempool_loaded via getmempoolinfo and /rest/mempool/info.json (Ben Woosley)

Pull request description:

  And use it to fix a race condition in mempool_persist.py:
  https://travis-ci.org/Empact/bitcoin/jobs/487577243

  Since e.g. getrawmempool returns errors based on this status, this
  enables users to test it for readiness.

  Fixes #12863

ACKs for commit effe81:
  MarcoFalke:
    utACK effe81f750
  jnewbery:
    utACK effe81f750

Tree-SHA512: 74328b0c17a97efb8a000d4ee49b9a673c2b6dde7ea30c43a6a2eff961a233351c9471f9a42344412135786c02bdf2ee1b2526651bb8fed68bd94d2120c4ef86

src/validation.h

@@ -142,7 +142,6 @@ extern CScript COINBASE_FLAGS;
 extern CCriticalSection cs_main;
 extern CBlockPolicyEstimator feeEstimator;
 extern CTxMemPool mempool;
-extern std::atomic_bool g_is_mempool_loaded;
 typedef std::unordered_map<uint256, CBlockIndex*, BlockHasher> BlockMap;
 extern BlockMap& mapBlockIndex GUARDED_BY(cs_main);
 extern Mutex g_best_block_mutex;
@@ -474,10 +473,10 @@ static const unsigned int REJECT_HIGHFEE = 0x100;
 CBlockFileInfo* GetBlockFileInfo(size_t n);
 
 /** Dump the mempool to disk. */
-bool DumpMempool();
+bool DumpMempool(const CTxMemPool& pool);
 
 /** Load the mempool from disk. */
-bool LoadMempool();
+bool LoadMempool(CTxMemPool& pool);
 
 //! Check whether the block associated with this index entry is pruned or not.
 inline bool IsBlockPruned(const CBlockIndex* pblockindex)