Implement idempotent REST publishing

[paddybyers]

This implements optional idempotent REST publishing as specified in https://docs.ably.io/client-lib-development-guide/features/#RSL1k:

(RSL1k) Idempotent publishing via REST is supported by populating the id attribute of Message instances passed to publish():
(RSL1k1) Idempotent publishing via library-generated Message ids is supported if idempotentRestPublishing (see TO3n) is enabled and one or more Message instances are passed to publish() and all Messages have an empty id attribute. The library generates a base id string by base64-encoding a sequence of at least 9 bytes obtained from a source of randomness. Each individual Message in the set of messages to be published is assigned a unique id of the form <base id>:<serial> (where serial is the zero-based index into the set).
(RSL1k2) Idempotent publishing via client-supplied Message ids is supported where a single Message is passed to publish() and it contains a non-empty id. The id is preserved on sending the message.

Prior to the 1.2 release, idempotent publishing via a library-generated id is disabled by default, but can be enabled via the idempotentRestPublishingclient option (TO3n).