Get ledger acquire work off the main thread and out of the master lock.

Prioritize ledger acquire work appripriately. This should help prevent the
server from becoming overly sluggish while acquiring ledgers. Still todo:
Finish all pending receive operations before sending out any data requests.

src/cpp/ripple/JobQueue.h

@@ -25,16 +25,17 @@ enum JobType
 	jtVALIDATION_ut	= 2,	// A validation from an untrusted source
 	jtPROOFWORK		= 3,	// A proof of work demand from another server
 	jtPROPOSAL_ut	= 4,	// A proposal from an untrusted source
-	jtCLIENT		= 5,	// A websocket command from the client
-	jtTRANSACTION	= 6,	// A transaction received from the network
-	jtPUBLEDGER		= 7,	// Publish a fully-accepted ledger
-	jtWAL			= 8,	// Write-ahead logging
-	jtVALIDATION_t	= 9,	// A validation from a trusted source
-	jtWRITE			= 10,	// Write out hashed objects
-	jtTRANSACTION_l	= 11,	// A local transaction
-	jtPROPOSAL_t	= 12,	// A proposal from a trusted source
-	jtADMIN			= 13,	// An administrative operation
-	jtDEATH			= 14,	// job of death, used internally
+	jtLEDGER_DATA	= 5,	// Received data for a ledger we're acquiring
+	jtCLIENT		= 6,	// A websocket command from the client
+	jtTRANSACTION	= 7,	// A transaction received from the network
+	jtPUBLEDGER		= 8,	// Publish a fully-accepted ledger
+	jtWAL			= 9,	// Write-ahead logging
+	jtVALIDATION_t	= 10,	// A validation from a trusted source
+	jtWRITE			= 11,	// Write out hashed objects
+	jtTRANSACTION_l	= 12,	// A local transaction
+	jtPROPOSAL_t	= 13,	// A proposal from a trusted source
+	jtADMIN			= 14,	// An administrative operation
+	jtDEATH			= 15,	// job of death, used internally
 
 // special types not dispatched by the job pool
 	jtPEER			= 24,