Log information about index version during startup

[e-dard]

Contributes to #9707.

This PR adds two new useful things to the logs:

1. During startup the index type in use for a shard will be logged when the shard is opened, using the index_version key. The two types available are: {"inmem", "tsi1"}.
2. The TSDB store will track all the different index types for all shards within all databases. Then, it will determine if there exist shards within a database that are running differing indexes, and log the frequency of each index type as a warning.

An example of the new logs (including a database with mixed index types) is as follows:

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2018-04-25T12:56:53.953773Z	info	Opened shard	{"log_id": "07gDtm50000", "service": "store", "trace_id": "07gDtmVW000", "op_name": "tsdb_open", "index_version": "tsi1", "path": "/Users/edd/.influxdb/data/_internal/monitor/1", "duration": "27.234ms"}
2018-04-25T12:56:53.955287Z	info	Opened shard	{"log_id": "07gDtm50000", "service": "store", "trace_id": "07gDtmVW000", "op_name": "tsdb_open", "index_version": "tsi1", "path": "/Users/edd/.influxdb/data/fooo/autogen/8", "duration": "20.021ms"}
2018-04-25T12:56:54.144353Z	info	Opened shard	{"log_id": "07gDtm50000", "service": "store", "trace_id": "07gDtmVW000", "op_name": "tsdb_open", "index_version": "inmem", "path": "/Users/edd/.influxdb/data/_internal/monitor/7", "duration": "253.250ms"}
2018-04-25T12:56:54.319992Z	info	Opened shard	{"log_id": "07gDtm50000", "service": "store", "trace_id": "07gDtmVW000", "op_name": "tsdb_open", "index_version": "inmem", "path": "/Users/edd/.influxdb/data/_internal/monitor/6", "duration": "428.724ms"}
2018-04-25T12:56:54.320059Z	warn	Mixed shard index types	{"log_id": "07gDtm50000", "service": "store", "inmem_count": 2, "tsi1_count": 1, "db_instance": "_internal"}
2018-04-25T12:56:54.320250Z	info	Open store (end)	{"log_id": "07gDtm50000", "service": "store", "trace_id": "07gDtmVW000", "op_name": "tsdb_open", "op_event": "end", "op_elapsed": "433.798ms"}
2018-04-25T12:56:54.320277Z	info	Opened service	{"log_id": "07gDtm50000", "service": "subscriber"}
2018-04-25T12:56:54.320285Z	info	Starting monitor service	{"log_id": "07gDtm50000", "service": "monitor"}
2018-04-25T12:56:54.320292Z	info	Registered diagnostics client	{"log_id": "07gDtm50000", "service": "monitor", "name": "build"}
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[stuartcarnie]

👌🏻