Make the transaction job queue limit adjustable:

The job queue can impose limits of how many jobs of a particular
type can be queued.

This commit makes the previously hard-coded limit associated with
transactions configurable by the server's operator. Servers that
have increased memory capacity or which expect to see an influx
of transactions can increase the number of transactions their
server will be able to queue.

This commit fixes #3556.

src/ripple/core/Config.h

@@ -173,6 +173,11 @@ public:
     // Compression
     bool COMPRESSION = false;
 
+    // Work queue limits
+    int MAX_TRANSACTIONS = 250;
+    static constexpr int MAX_JOB_QUEUE_TX = 1000;
+    static constexpr int MIN_JOB_QUEUE_TX = 100;
+
     // Amendment majority time
     std::chrono::seconds AMENDMENT_MAJORITY_TIME = defaultAmendmentMajorityTime;