Rename PLTSBlock -> PLTSBlockBase
Introduce PLTSActiveBlock
REBASEME: move all the code to ActiveBlock
REBASME: move lvl, next and prev to BaseBlock
REBASEME: move capacity, offset and data to BaseBlock
REBASEME: avoid using this-> to access members of base class
REBASEME: extend preliminary code for reducing block size"
DOC
add LastActiveBlock