Support for .(hlog) histogram log output

[filipecosta90]

cc @giltene

As per Java documentation:

Histogram log format:
A histogram log file consists of text lines. Lines beginning with the "#" character are optional and treated as comments. Lines containing the legend (starting with "Timestamp") are also optional and ignored in parsing the histogram log. All other lines must be valid interval description lines. Text fields are delimited by commas, spaces.

A valid interval description line contains an optional Tag=tagString text field, followed by an interval description.

A valid interval description must contain exactly four text fields:

    StartTimestamp: The first field must contain a number parse-able as a Double value, representing the start timestamp of the interval in seconds.
    intervalLength: The second field must contain a number parse-able as a Double value, representing the length of the interval in seconds.
    Interval_Max: The third field must contain a number parse-able as a Double value, which generally represents the maximum value of the interval histogram.
    Interval_Compressed_Histogram: The fourth field must contain a text field parse-able as a Base64 text representation of a compressed HdrHistogram.

The log file may contain an optional indication of a starting time. Starting time is indicated using a special comments starting with "#[StartTime: " and followed by a number parse-able as a double, representing the start time (in seconds) that may be added to timestamps in the file to determine an absolute timestamp (e.g. since the epoch) for each interval.