Replace FindLatestBefore used by importmuti with FindEarliestAtLeast.

In spite of the name FindLatestBefore used std::lower_bound to try
 to find the earliest block with a nTime greater or equal to the
 the requested value.  But lower_bound uses bisection and requires
 the input to be ordered with respect to the comparison operation.
 Block times are not well ordered.

I don't know what lower_bound is permitted to do when the data
 is not sufficiently ordered, but it's probably not good.
 (I could construct an implementation which would infinite loop...)

To resolve the issue this commit introduces a maximum-so-far to the
 block indexes and searches that.

For clarity the function is renamed to reflect what it actually does.

An issue that remains is that there is no grace period in importmulti:
 If a address is created at time T and a send is immediately broadcast
 and included by a miner with a slow clock there may not yet have been
 any block with at least time T.

The normal rescan has a grace period of 7200 seconds, but importmulti
 does not.

src/chain.cpp

@@ -61,10 +61,10 @@ const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
     return pindex;
 }
 
-CBlockIndex* CChain::FindLatestBefore(int64_t nTime) const
+CBlockIndex* CChain::FindEarliestAtLeast(int64_t nTime) const
 {
     std::vector<CBlockIndex*>::const_iterator lower = std::lower_bound(vChain.begin(), vChain.end(), nTime,
-        [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTime() < time; });
+        [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTimeMax() < time; });
     return (lower == vChain.end() ? NULL : *lower);
 }