Add timer start to SAVEQUERIES data

[Rarst]

Currently $wpdb->queries will capture elapsed query times, but lacks the start time point for each query.

Adding this data will be helpful for performance tools to determine the distribution of queries in a page load and to correlate queries with containing operations.

[johnbillion]

In 42789:

Database: Add the query start time to logged query data.

This allows debugging plugins to plot the time at which queries were executed.

Props Rarst for initial patch.

Fixes #43315

[johnbillion]

@since needs updating

[SergeyBiryukov]

In 44345:

Docs: Update @since tag for the addition of query start time to logged query data.

Fixes #43315.

[desrosj]

This does not appear to have been backported to the 5.0 branch, so updating the milestone to be correct.

[desrosj]

This was mentioned in a 5.1 dev note: ​https://make.wordpress.org/core/2019/01/23/miscellaneous-developer-focused-changes-in-5-1/