Separate multiple values of same tag with comma

[bocharovf]

Which problem is this PR solving?

Resolves #98

Short description of the changes

Store multiple different values of the same tag in a single key separated by comma. It allows us to find trace by any of the tag value (not only first one).

PROS:

1. Ability to search any tag value
2. Backward compability (no schema changes or update required).

CONS:

1. Search for tag value containing comma will be incorrect (could be solved by parametrizing separator)