WL#4394: Changes in the storage engine API versioning for InnoDB

Affects: Server-9.x   —   Status: Un-Assigned   —   Priority: Medium

Currently the version of the storage engine API is created from the server
version - and it means that a storage engine plugin can only be loaded in the 
exactly the same minor server version that is was built for.

It's inconvenient for storage engine vendors who don't have the resources to
build a new plugin binary for every MySQL release. Additionally, certain storage
engine plugins may use only a subset of the API, and vendors don't want changes
in the unused parts of the API affect the load-ability of the plugin.

We'll fix it as follows:

1. we will start using icheck to control all changes to the storage engine API
2.by studying the generated .ic file we will get a fairly good idea of what
structures and classes the SE API currently consists of
3. Then we will define what subset of the SE API is currently used by InnoDB
4. If it will happen to be notably smaller than the "full" SE API (as defined by
.ic file) we will implement a workaround so that InnoDB plugin version could be
loaded in the server with mismatching SE API version.