implement batch upload for /instances, /items and /holdings

The batch endpoints should work as follows:

• the client bundles a set of records into a single JSON array structure called a "batch". It's up to the client to split the entire collection of records into multiple batches.
• the byte size of the of the batch is determined by the client through the Content-Length header with the upper limit decided on the server-side by the module. If the upper limit is reached the module will report a 413 Payload Too Large error to the client. The server may also handle requests with missing or wrong Content-Length and report this error during processing when the limit is reached.
• the DB transaction is opened when the processing of a batch is started and is committed when all records from the batch have been successfully stored in the DB at which point the server responds with 201 Created.
• in case of errors, the server rollbacks the transaction and responds with 422 Unprocessable Entity (all-or-nothing approach)
• the server must use a method to store the records in the DB efficiently: e.g the saveBatch method in the RMB PostgresClient, based on the old vertx pg-mysql driver, or batch from https://vertx.io/docs/vertx-pg-client/java/

Note on the API

mod-inventory-storage already includes partial batch update API called inventory-storage-batch. The semantics of this API are not entirely compatible with what has been proposed above. The new API should be introduced as a new interface, independent of this existing API for backwards compatibility reasons. The new interface should be called inventory-storage-batch-sync (to differentiate from an async API that may be introduced at a later stage)

Not implemented

Content-Length/413 Payload Too Large and using a streaming method without loading the complete batch into mod-inventory-storage memory requires RMB support that doesn't exist yet. Therefore it cannot be implemented within the scope of this issue. It this has been split into RMB-505.
The implementation uses PostgresClient.saveBatch that does not support streaming.