Connect to InfluxDB via a Unix Domain Socket

## Feature Request

### Proposal:

Add support for connecting to InfluxDB over a unix domain socket

### Current behavior:

We can connect to InfluxDB via UDP or HTTP(S) over TCP but not UDS. InfluxDB also supports HTTP over UDS.

See : https://github.com/influxdata/influxdb/pull/7135
https://github.com/influxdata/telegraf/issues/3366

### Desired behavior:

Be able to talk to InfluxDB on the same host via UDS.

### Use case:

1) Improved security
2) Reduce TCP overhead for hosts running the TICK stack on the same box and have limited resources

Start by tracing Kapacitor's existing InfluxDB connection paths for UDP and HTTP(S), then compare the requested Unix domain socket behavior with the linked InfluxDB and Telegraf references. Done means same-host connections to InfluxDB work over UDS while existing UDP and TCP behavior remains available.

Written by the indexing model from the issue text.