Sleep for 200us before each ActivateBestChainStep call

This should lower the main thread's likelihood to immediately reacquire
cs_main after dropping it, which should help ThreadNotifyWallets and the
RPC methods to acquire cs_main more quickly.

Ported from ZEC commit e2cd1b761fe556bc6d61849346902c3611530307

src/main.cpp

@@ -33,6 +33,7 @@
 #include "net.h"
 #include "netmessagemaker.h"
 #include "pow.h"
+#include "time.h"
 #include "script/interpreter.h"
 #include "txdb.h"
 #include "txmempool.h"
@@ -4333,7 +4334,15 @@ bool ActivateBestChain(bool fSkipdpow, CValidationState &state, CBlock *pblock)
     CBlockIndex *pindexMostWork = NULL;
     const CChainParams& chainParams = Params();
     do {
-        boost::this_thread::interruption_point();
+        // Sleep briefly to allow other threads a chance at grabbing cs_main if
+        // we are connecting a long chain of blocks and would otherwise hold the
+        // lock almost continuously.  This helps
+        // the internal wallet, if it is enabled, to keep up with the connected
+        // blocks, reducing the overall time until the node becomes usable.
+        //
+        // This is defined to be an interruption point.
+        // <https://www.boost.org/doc/libs/1_72_0/doc/html/thread/thread_management.html#interruption_points>
+        boost::this_thread::sleep_for(boost::chrono::microseconds(200));
 
         if (ShutdownRequested())
             break;