Parse timestamp metrics from build log

from https://gitlab.com/gitlab-org/gitlab-runner/issues/2505#note_40484261 , build logs will be annotated with markers like: \rsection_start:1505492007:clone\r and \rsection_end:1505492107:clone\r

1. We need to create a new table: ci_build_trace_sections with: byte_start, byte_end, date_start, date_end, name,
2. Parse markers once build is finished,
3. Store markers in a separate table.

This allows us later to:

1. Create analytics how long each section took, or how long jobs in pipeline spend on cloning,
2. Create a timeline of whole pipeline showing overlap between sections,
3. Make collapsable sections as part of job, or sections to be separate traces displayed,
4. Make data accessible via API or be pushed to other systems for accounting (via webhooks?).

Let's keep the marker implementation details on https://gitlab.com/gitlab-org/gitlab-runner/issues/2505

cc: @ayufan @bikebilly