Support for float timestamp for logs

[oplehto]

In the logs panel and logs explore, if the first column is a float then it should be considered a timestamp with subsecond precision (similar to the UInt64 special case for Unix timestamps).

[oplehto]

Thining a bit further, the most versatile way would be the following:

• The timestamp column should be obtained from the "Query Settings" tab if its set there
• The "Query Settings" timestamp selector should support also Float* and UInt64 based timestamps (this would also benefit timeseries and table panes)

[Slach]

@oplehto when use UInt64, shall we define how many right pad digits will use for fractional seconds? or just need to define 3 right digits always and use is as milliseconds?