Deprecate old style floats in history table in 6.4, remove in 7.0

New type of floats ("double precision") was introduced in 5.0 for decimal values in history and trends but up to this point Zabbix supported both old and new types.

It's time to deprecate old types in 6.4 because:

• some code may require knowledge of what type is used and this is an unnecessary maintenance overhead
• the old type is already deprecated by MySQL

The deprecation can be done as:

• A note in upgrade notes for 6.4, a paragraph in upgrade instructions
• A warning about deprecation in the log file
• Optionally, system information widget / report can give a warning about deprecation

In 7.0 Zabbix should not support the old floats altogether.