A sharp drop in interest rates last week suddenly made millions more borrowers eligible to refinance their mortgages.  With the average rate on the 30-year fixed now close to 4 percent, 4.9 million borrowers could likely qualify for a refinance that could reduce their interest rates by at least three quarters of a percentage point, according to Black Knight, a mortgage data and analytics company. That is a nearly 50 percent increase in the size of that population in a single week.  Continued…