Extend log processing tools to quantify _phases_ of execution

[jberthold]

For performance optimisations, it would be important to know how much time which component, or which phase of request processing (e.g., matching, equation-based simplification, constraint checking, smt-solver interaction, internalisation) consumes.
This information could be gathered from a (complete!) context log file with timestamps, by grouping the time difference for suitable "spans" of log entries into categories and accumulating their time over an entire log file (or per request).