Improve uses of boost multi_index_container

[guilhermelawless]

• Switched to using a sequence index for dropped_elections_cache, confirmed_set, rolled_back, and block_arrival. This allows using push_back + pop_front with no sorting, and removing the timepoint in some cases where it was only used for ordering.
• Replaced some insert with emplace.
• Removed the <boost/multi_index/random_access_index.hpp> header where not necessary.
• Added index tags everywhere. Some places had issues with not using the correct index.
• Formatting, and enclosing declarations in clang-format off to be more readable. Was originally done in rep_crawler.