WL#3952: Add @@optimizer_switch variable

Affects: Server-6.0   —   Status: Complete

Add @@optimizer_switch variable that will allow to turn off individual
1. Rationale
We need the ability to turn off individual optimizations. This is necessary
for easier testing and meaningful comparative benchmarks.

2. Solution description
We'll introduce a new session/global variable, named


The value of the variable is a comma-separated list of optimizations that
are turned off. The underlying structure is a set (bitmap). Elements of the
list will be named after this pattern:


In scope of this WL entry we will implement switch-off functionality for two
optimizations: semi-join (WL#2980) and materialization (WL#1110).

3. System variable checklist answers

- Purpose 
  A set of disabled optimizations.

- Name
  - System variable name:  @@optimizer_switch
  - Name for the script "configure": none (not settable)
  - Name for the configuration file: none (not settable)
  - Name for the command-line option: none (not settable)

- Allowed values
  - Type: Comma-separated list of mnemonic names.
          Currently only two names are possible: 
  - Range of proper values: {not a range}
  - Default value: {empty string}

- Scope: both session and global
- Read-only or dynamic in each scope: Dynamic 
- Dependencies: none
- Relation to stored programs: it is a "dynamic variable"
- Relation to replication: No auto-replication.

Follow the approach used in @@sql_mode.