Merge readerWriters counter and referres counter in disk cache pointers into one

[andrii0lomakin]

Currently, we use reader-writer counters to check which pages exclusively belong to the write cache. Once the memory limit is reached, these pages will be the first candidates to flush on the disk. Also, a reference counter is used to evict pages allocated in direct memory if there are no more references to the pages. They perform similar functions and can be replaced by a single reader-writer counter.