rpc: Expose g_is_mempool_loaded via getmempoolinfo and /rest/mempool/info.json

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.

doc/REST-interface.md

@@ -101,6 +101,7 @@ $ curl localhost:18332/rest/getutxos/checkmempool/b2cdfd7b89def827ff8af7cd9bff76
 
 Returns various information about the TX mempool.
 Only supports JSON as output format.
+* loaded : (boolean) if the mempool is fully loaded
 * size : (numeric) the number of transactions in the TX mempool
 * bytes : (numeric) size of the TX mempool in bytes
 * usage : (numeric) total TX mempool memory usage