database: Remove tx log in favor of new ldb txns.
database: Remove tx log in favor of new ldb txns.

This removes the intermediate transaction log that was introduced as a
part of the database cache as a workaround for leveldb batches causing
massive memory usage spikes in favor of the recently introduced leveldb
transaction interface which no longer has the memory usage issues.

This approach is preferred because it can avoid the extra memory needed
for the transaction log and therefore all of the intermediate states as
well.  As a result, the default cache size has been doubled since it
equals roughly the same amount of overall memory usage and the flush
interval has been raised as well.

qchain/btcd,qchain/btcd