New select DB command to RedisArray - Added retry delay on reconnect

Added the possibility to delay each reconnection attempt, including a
random factor to prevent several or many concurrent connections from
trying to reconnect at the same time.
Added the select command to RedisArray to select a DB on every
connections in one instruction.
Also, fixed a compiler warning:
redis_array_impl.c:1115:15: warning: incompatible pointer types
assigning to 'zval **' (aka 'struct _zval_struct **') from 'zval
**(*)[2]' [-Wincompatible-pointer-types]