limit max thread count to 126

When the number of threads is higher than 126 the number of LMDB locks
is exhausted and `ArmoryDB` dies with the error:

```
MDB_READERS_FULL: Environment maxreaders limit reached
```

See:
lmdbjava/lmdbjava#65 (comment)

Replace the uses of `std::thread::hardware_concurrency()` with a new
function `MAX_THREADS()` which limits the number of threads to `126`.

This is a temporary fix, the real fix would be making sure the number of
LMDB locks is sufficient.

However, a configured value for the maximum number of threads would also
be useful.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

cppForSwig/BDM_Server.cpp

@@ -1848,7 +1848,7 @@ void Clients::init(BlockDataManagerThread* bdmT,
    unsigned innerThreadCount = 2;
    if (BlockDataManagerConfig::getDbType() == ARMORY_DB_SUPER &&
       BlockDataManagerConfig::getOperationMode() != OPERATION_UNITTEST)
-      innerThreadCount = thread::hardware_concurrency();
+      innerThreadCount = MAX_THREADS();
    for (unsigned i = 0; i < innerThreadCount; i++)
    {
       controlThreads_.push_back(thread(innerthread));

cppForSwig/BlockDataManagerConfig.cpp

@@ -20,6 +20,20 @@
 
 using namespace std;
 
+size_t MAX_THREADS()
+{
+   size_t cpu_threads = std::thread::hardware_concurrency();
+
+   if (!cpu_threads)
+      return 1;
+   // there are only 126 LMDB locks by default
+   // FIXME: need to increase number of LMDB locks
+   else if (cpu_threads > 126)
+      return 126;
+
+   return cpu_threads;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 //
 // NodeStatusStruct

cppForSwig/BlockDataManagerConfig.h

@@ -28,6 +28,8 @@
 #define DEFAULT_ZCTHREAD_COUNT 100
 #define WEBSOCKET_PORT 7681
 
+size_t MAX_THREADS();
+
 class BitcoinP2P;
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -66,7 +68,7 @@ struct BlockDataManagerConfig
    bool customBtcPort_ = false;
 
    unsigned ramUsage_ = 4;
-   unsigned threadCount_ = std::thread::hardware_concurrency();
+   unsigned threadCount_ = MAX_THREADS();
    unsigned zcThreadCount_ = DEFAULT_ZCTHREAD_COUNT;
 
    std::exception_ptr exceptionPtr_ = nullptr;

cppForSwig/Server.cpp

@@ -234,7 +234,7 @@ void WebSocketServer::start(BlockDataManagerThread* bdmT,
       instance->clientInterruptThread();
    };
 
-   unsigned parserThreads = thread::hardware_concurrency() / 4;
+   unsigned parserThreads = MAX_THREADS() / 4;
    if (parserThreads == 0)
       parserThreads = 1;
    for (unsigned i = 0; i < parserThreads; i++)
@@ -1038,4 +1038,4 @@ void ClientConnection::processAEADHandshake(BinaryData msg)
 void ClientConnection::closeConnection()
 {
    run_->store(-1, memory_order_relaxed);
-}
+}

cppForSwig/ZeroConf.cpp

@@ -290,7 +290,7 @@ void ZeroConfContainer::preprocessZcMap(
    };
 
    vector<thread> parserThreads;
-   for (unsigned i = 1; i < thread::hardware_concurrency(); i++)
+   for (unsigned i = 1; i < MAX_THREADS(); i++)
       parserThreads.push_back(thread(parserLdb));
    parserLdb();
 
@@ -2111,4 +2111,4 @@ bool ZcUpdateBatch::hasData() const
       return true;
 
    return false;
-}
+}