performance and readability refactor

[boris-dimitrov]

This change reduces DB load time from 104 to 14 seconds, speeds up queries, refactors the DB loading and parsing loop for better readability and to facilitate future performance work.

• report error for truncated read sequence at EOF

• reserve memory for SNPS and MMERS arrays

• replace unordered_map with array for 3x speedup

• refactor DB loading; correctly loads first and last lmer

It also retains the functionality of the recent improvements to wildcard handling in master.

[boris-dimitrov]

This is now ready for review and to be merged into master.