Implement redis commands APPEND, properly implement the SET command, implement SET command variations (e.g. SETNX and so on)

[danielealbano]

Implement redis commands APPEND, properly implement the SET command, implement SET command variations (e.g. SETNX and so on)

Below the full list of commands to implement

Command	Notes
✔ APPEND
✔ COPY	Missing DB parameter
✔ DBSIZE
✔ DECR
✔ DECRBY
✔ DEL
✔ EXISTS
✔ EXPIRE
✔ EXPIREAT
✔ EXPIRETIME
✔ FLUSHDB	Missing ASYNC parameter
✔ GET
✔ GETDEL
✔ GETEX
✔ GETRANGE
✔ GETSET
✔ HELLO	Missing AUTH and SETNAME parameters
✔ INCR
✔ INCRBY
✔ INCRBYFLOAT
✔ KEYS
✔ LCS	Missing IDX, MINMATCHLEN and WITHMATCHLEN parameters
✔ MGET
✔ MSET
✔ MSETNX	Currently buggy, might hang the server
✔ PERSIST
✔ PEXPIRE
✔ PEXPIREAT
✔ PEXPIRETIME
✔ PING
✔ PSETEX
✔ PTTL
✔ QUIT
✔ RANDOMKEY
☐ RENAME
☐ RENAMENX
✔ SCAN	Missing TYPE parameter
✔ SET
✔ SETEX
✔ SETNX
✔ SETRANGE
✔ SHUTDOWN
✔ STRLEN
✔ SUBSTR
✔ TOUCH
✔ TTL
☐ TYPE
✔ UNLINK