tsdb store: cannot allocate memory

Trying to start the influxd server with `sudo influxd` (or `influxd`) gives
`open server: open tsdb store: cannot allocate memory`

Running Influx 1.5.2 on Ubuntu 18.04 LTS on WSL (Windows 10 1803). Reinstall fixed the problem, but as the database gets larger, the same issue repeats. Looking at other issues on GitHub, I wondered if it was a RAM issue, but I have over 5GB of ram (while trying to start the server), and the influxdb folder is just over 143MB.

Start by reproducing the `influxd` startup failure on Ubuntu 18.04 under WSL, using the reported growing database and the `tsdb store` error as the entry points. Compare startup behavior as the database grows and trace the memory-allocation failure; done means the cause is identified and a reproducible fix or actionable diagnosis is established.

Written by the indexing model from the issue text.