SQL cursor should be protected via token hold from concurrent reads / writes

There are currently some holds in [functions that call `SQL`](https://github.com/Dyalog/DCMS/blob/4b88fa207f9e7b7855d75280cd46f9f819d6f911/APLSource/CRUD/Create.aplf#L56) to do database operations, but putting holds inside SQL would better ensure reads and writes are synchronous. For example, [`QUERY.BuildCache`](https://github.com/Dyalog/DCMS/blob/4b88fa207f9e7b7855d75280cd46f9f819d6f911/APLSource/QUERY/BuildCache.aplf#L9) currently does SQL reads when another thread could be in the middle of updates.

Start by reading the existing holds around SQL in APLSource/CRUD/Create.aplf and the SQL reads in APLSource/QUERY/BuildCache.aplf. Trace the SQL entry point and determine how a token hold should cover cursor reads and writes; done means concurrent database operations are synchronous without leaving the existing callers unprotected.

Written by the indexing model from the issue text.