Move reqst_machine into mtd.cc, where it belongs.
Perfstat: Remove mark_delta_created/removed.

Use threadcounters instead.
Remove unused code.
Simplify kvthread allocation functions.
Use value_array specific functions to alloc/free columns.
The memtag is a mandatory argument.
Use one memtag for all value data.

Two types of value never coexist anyway.
Make allocate threadallocation arguments mandatory.
Remove allocationtag.

It is now redundant with memtag: (memtag == memtag_value)
== (allocationtag == ta_data).
Track memory statistics through kvthread.

Not Perf::stat. This loses some functionality (that in
Perf::stat::print).
Disable GETSTATS.
Track GC statistics through threadcounters.

Not perfstat. Note: The Perf::stat version was broken because of
a misspelled preprocessor symbol!
Remove Perf::stat.

Leave the code around, though, in case we want to refer to it.