Add structures for exposing thread events and operations.

Summary:
Add structures for exposing events and operations.  Event describes
high-level action about a thread such as doing compaciton or
doing flush, while an operation describes lower-level action
of a thread such as reading / writing a SST table, waiting for
mutex.  Events and operations are designed to be independent.
One thread would typically involve in one event and one operation.

Code instrument will be in a separate diff.

Test Plan:
Add unit-tests in thread_list_test
make dbg -j32
./thread_list_test
export ROCKSDB_TESTS=ThreadList
./db_test

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: rven, jonahcohen, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29781