Fixed PXB-2858 Prepare of full and incremental backup uses the same a…

[altmannmarcelo]

…mount of buffer pool memory with memory estimation

https://jira.percona.com/browse/PXB-2858

Problem: When preparing incremental backups, we call read_metadata twice. First time we read metadata file from incremental dir, but second time we read it from target-dir (full base backup). The second call was replacing redo_memory & redo_frames variable.

Fix: Adjusted code to use a variable that gets populated conditionally in case we are doing a prepare on full or inc directory.