Clean up directory layout

[apavlo]

We meant through our list of directories last month and decided to move some directories inside of each other. Here are targets:

• 'src/container' -> 'src/common/container'
• 'src/util' -> 'src/common/util'
• 'src/threadpool' -> 'src/common/threadpool'
• 'src/statistics' -> 'src/storage/statistics'
• Rename 'concurrency' -> 'transaction'
• Rename 'src/settings' to 'src/setting'
• Rename 'src/statistics' to 'src/stat'
• 'src/gc' -> 'src/storage/gc'
• 'src/trigger' + 'src/expression' + 'src/function' into 'src/type' or 'src/sql' or 'src/query' ????
• 'src/binder' + 'src/parser' + 'src/optimizer' into 'src/planner'. Current files in 'src/planner' to 'src/planne/plan'

We will need to find the right time to do this, since it will break a bunch of PRs. We may want to do this incrementally.