Improve thread statics performance

[jkotas]

• Disable code to handle multiple appdomains
• Use more efficient object array accessor

[jkotas]

The following called in a loop is ~17% faster:

[ThreadStatic]
static object o;

[MethodImpl(MethodImplOptions.NoInlining)]
static object foo() { return o; }

[stephentoub]

The following called in a loop is ~17% faster:

That's on top of the ~20% improvement from your previous PR? That's great!