Look into reusing open map files during log recovery.

In #458 log recovery was modified in such a way that sorted recovery logs are opened and closed multiple times during recovery. This results in the sorted log indexes being read into memory multiple times. It may be faster to open the sorted logs once and reuse/reseek them. Need to look into how long it takes to open a sorted log and read its index. The changes for #881 may help with the performance a bit because it filters out sorted logs that do not define a tablet in the first step.