Add support set the max db scan count when first connect the redis server.

[lefay1982]

RDM Version: 0.8.7+
Environment (OS name and version): win7
Redis Server Version: 2.8.19 Aliyun

the Aliyun's Redis set the db size is 256, I can't modify, but I only use less than 10 db.
the RDM 0.8.7+ connect the redis server from db0 to db256. the open time is very long.
the RDM 0.8.6- only scan 16 db.
can you add custom setting Max-DB-Scan, so I can only scan 10 db, not 256.

the System log:

........... from select 0

2016-09-14 01:37:41 : Connection: AliKv01 127.0.0.1 6379 > [runCommand] select 59
2016-09-14 01:37:41 : Connection: AliKv01 127.0.0.1 6379 > Response received : +OK

2016-09-14 01:37:41 : Connection: AliKv01 127.0.0.1 6379 > [runCommand] select 60
2016-09-14 01:37:41 : Connection: AliKv01 127.0.0.1 6379 > Response received : +OK

...... to select 256