Sonic v1.2.0

• Fixed a rare deadlock occuring when 3 concurrent operations get executed on different threads for the same collection, in the following timely order: PUSH then FLUSHB then PUSH [@valeriansaliou, d96546b].
• Reworked the KV store manager to perform periodic memory flushes to disk, thus reducing startup time [@valeriansaliou, 6713488].
• Stop accepting Sonic Channel commands when shutting down Sonic [@valeriansaliou, #131].
• Introduced a server statistics INFO command to Sonic Channel [@valeriansaliou, #70].
• Added the ability to disable the lexer for a command with the command modifier LANG(none) [@valeriansaliou, #108].
• Added a backup and restore system for both KV and FST stores, which can be triggered over Sonic Channel with TRIGGER backup and TRIGGER restore [@valeriansaliou, #5].
• Added the ability to disable KV store WAL (Write-Ahead Log) with the write_ahead_log option, which helps limit write wear on heavily loaded SSD-backed servers [@valeriansaliou, #130].