tail plugin: Rename DSType from Latency to Distribution.

tail plugin: Rename DSType from Latency to Distribution.

This is a more flexible naming owing to the fact that percentiles and other
distribution parameters are interesting for a variety of metrics, not just
latencies.

The config handling is now completely in src/utils_latency_config.c so
that other plugins, such as the cURL plugin, can easily reuse the module
with consistent config handling.