fix issue #5598 misleading output on import

[qblays]

The line

aleth/aleth/main.cpp

Line 832 in 2fe1bec

tuple<ImportRoute, bool, unsigned> r = web3.ethereum()->syncQueue(10);

always return zero number of synced blocks. It happens because BlockQueue::drain can take m_lock (and it always does) before BlockQueue::verifierBody. So

aleth/libethereum/BlockQueue.cpp

Line 426 in 2fe1bec

o_out = m_verified.dequeueMultiple(min<unsigned>(_max, m_verified.count()));

tries to extract from empty m_verified queue. The solution is to wait until all unverified blocks gone and then sync

[codecov-io]

Codecov Report

Merging #5878 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5878      +/-   ##
==========================================
+ Coverage   63.94%   63.95%   +0.01%     
==========================================
  Files         365      365              
  Lines       31098    31101       +3     
  Branches     3441     3441              
==========================================
+ Hits        19885    19891       +6     
+ Misses       9980     9979       -1     
+ Partials     1233     1231       -2