Refactor RaftGroupMetadata to avoid keeping unnecessary TableInfo objects in memory

[hectorgcr]

Currently , RaftGroupMetadata class has a unique_ptr to a TableInfo object. The problem is that this object sometimes needs to be kept alive to avoid race conditions, so when an AlterTable or a remote bootstrap happens, a TableInfo objects is stored in a vector called old_tables just in case someone is referencing it through a raw pointer. We should redesign this to use shared_ptrs instead.

Another problem is that the RaftGroupMetadata class provides reference access to fields from the main's table TableInfo object, e.g. schema(), partition_schema(), so if we make the change described above, we also need to make sure that these methods don't return a const reference because by doing so, we won't increase the ref count of the TableInfo object, and the caller could end up with a dangling reference if the TableInfo object is suddenly destroyed. We should solve this by returning shared_ptrs to such fields from TableInfo. These fields could also be changed to be shared_ptrs, or we could construct these shared_ptrs on the fly by using the aliasing shared_ptr constructor:

template< class Y >
shared_ptr( const shared_ptr<Y>& r, element_type* ptr ) noexcept;