client: make gc step non-static

All connections has GC of input buffer, and step counter is static now.
It breaks multithreaded scenarios since several connections can be use
concurrently from several threads, so let's move the GC step counter
right to `Connection` class.

Part of #110

Author: drewdzzz

src/Client/Connection.hpp

@@ -250,6 +250,7 @@ class Connection
 private:
 	ConnectionImpl<BUFFER, NetProvider> *impl;
 	static constexpr size_t GC_STEP_CNT = 100;
+	size_t gc_step = 0;
 
 	template <class T>
 	rid_t insert(const T &tuple, uint32_t space_id);
@@ -521,8 +522,7 @@ template<class BUFFER, class NetProvider>
 static void
 inputBufGC(Connection<BUFFER, NetProvider> &conn)
 {
-	static int gc_step = 0;
-	if ((gc_step++ % Connection<BUFFER, NetProvider>::GC_STEP_CNT) == 0) {
+	if ((conn.gc_step++ % Connection<BUFFER, NetProvider>::GC_STEP_CNT) == 0) {
 		LOG_DEBUG("Flushed input buffer of the connection %p", &conn);
 		conn.impl->inBuf.flush();
 	}