increase 'float' precision & data length

currently items that store their values as "float" actually store them as fixed decimal values of precision 16,4.
this causes several problems :

a) storing network traffic as bits per second on a fast and heavily utilised interface overflows the whole number part;
b) fping that zabbix uses for icmp checks returns values as milliseconds with two decimal places. zabbix inserts seconds in the database, and as we're limited to 4 decimal places there, we lose one number.

current suggestion would be to bump these values to 26,6 or 28,8

any place where server does value length check should be reviewed, too