innodb_expand_fast_index_creation should be optional

[Alexey Kopytov]

The server-side functionality from innodb_expand_fast_index_creation.patch should be made optional for a number of reasons:

1. We promise the same behavior as compared to the upstream server unless Percona-specific features are explicitly enabled by corresponding options.

2. Fast index creation may actually be slower than the default method in some cases. (e.g. when the data fits in the buffer pool and the merge buffer is too small, see https://blueprints.launchpad.net/percona-server/+spec/tunable-buffer-size-for-merge-sort)

3. There are upstream server bugs that make fast index creation inefficient in even more cases, for example https://bugs.launchpad.net/bugs/810571

We should have an option controlling whether fast index creation is used, and it should be disabled by default.

[Alexey Kopytov (akopytov)]

Quoting revision comments for RN and documentation updates:

"
This fix makes all functionality provided by
innodb_expand_fast_index_creation.patch disabled by default.

It can be enabled by using a new system variable,
innodb_expand_fast_index_creation, that has both global or session
scopes, and can also be specified in a configuration file or on the
command line. The variable is of the boolean type and is FALSE by
default. The session value of TRUE enables optimizations and
improvements provided by the patch. The variable also has no effect when
fast index creation is disabled with the fast_index_creation option
provided by control_online_alter_index.patch.
"

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2688