Fixed pruning logic

- Erase an element from m_blocksByHeight only if it's empty
- Remove blocks from unconnected alternative chains too when they get old enough (4xPPLNS window)

src/pool_block.cpp

@@ -52,6 +52,7 @@ PoolBlock::PoolBlock()
 	, m_invalid(false)
 	, m_broadcasted(false)
 	, m_wantBroadcast(false)
+	, m_localTimestamp(time(nullptr))
 {
 	uv_mutex_init_checked(&m_lock);
 
@@ -114,6 +115,8 @@ PoolBlock& PoolBlock::operator=(const PoolBlock& b)
 	m_broadcasted = b.m_broadcasted;
 	m_wantBroadcast = b.m_wantBroadcast;
 
+	m_localTimestamp = time(nullptr);
+
 	if (lock_result == 0) {
 		uv_mutex_unlock(&b.m_lock);
 	}