Normalize LIB nomenclature

[heifner]

There are a number of different things we call lib or last irreversible block.

• Savanna block's core.last_final_block_num also known as irreversible_blocknum
• Legacy block's dpos_irreversible_blocknum also known as irreversible_blocknum
• fork database root
• fork database pending_savanna_lib_id
• block log head (might not exist if running with no block log)

Going forward we would like to use lib or last irreversible block in logs and documentation to refer to irreversible_blocknum which is the lib in respect to a given block. In locations where fork database root or fork database pending savanna lib is required they should be referred to fork_db_root/froot and fork_db_pending_lib. The block log head should be referred to simply as the block log head.

This issue is to examine and cleanup any log messages, method names, and comments in the code base to correspond to this new nomenclature.