Use timeseries module's find_entries_count() function to count database entries

[MukuFlash03]

As requested in this issue, added code implementation for counting the total number of documents in the timeseries databases.

This PR along with the issue mentioned above, under e-mission/e-mission-server contains detailed information acquired during the implementation process.

The functionfind_entries_count()can be accessed via the abstracted interface for timeseries using abstract_timeseries.
It takes in these parameters: key_list, time_query, geo_query, extra_query_list.
It supports both single and multiple keys passed as a list.

For sample usage and test cases, please refer to TestTimeseries.testFindEntriesCount