Implement a set of commands

[danielealbano]

This PR implements a set of PRs or the issue #126 .

The following commands have been implemented in the following PR

Command	Notes
DBSIZE
EXISTS
FLUSHDB	Missing ASYNC
GETSET
LCS	Missing IDX, MINMATCHLEN and WITHMATCHLEN
MSETNX	Currently buggy, might hang the server
PTTL
TTL

In addition:

• the DEL command has been refactored to take advantage of the implemented shortcuts
• the HELLO command now reports an error if the AUTH or SETNAME parameters are used as they are currently unsupported