WL#9468: Improve string usage in DD code

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

Incrementally improve the usage of strings in the DD code. Currently the DD code 
makes extensive use of std::string, which is safe (compared to const char *), but 
allocates directly on the system heap (bypasses performance schema 
instrumentation) and may throw exceptions.

User Documentation

Code cleanup. No user documentation required.
Changes will be done incrementally in the following stages (more stages may be 
added later):

1. Introduce a typedef dd::string_type for all string usage in the DD code. 
Initially this typedef will be an alias for std::string. This will simplify the 
process of trying out different string implementations.

2. Create a template for a stateless allocator which can be instantiated with 
closures that use different performance schema memory keys. (Since the PFS key 
not a compile time constant, it cannot be given directly as a template argument, 
but must be provided by a closure or callable type).

3. Create a PFS key for DD string usage.

4. Change the typedef in 1. to an instantiation of std::basic_string using the 
allocator from 2.