Add long transaction support to wtperf.

Enabled by adding ops_per_txn to thread configuration groups.
Add the code changes tfor wtperf ops_per_txn.
Update wtperf documentation.
Fixup LSM drop:
* If a bloom drop succeeded but the chunk drop failed, we could skip the
  metadata update. Which would lead to a confused LSM tree after restart.
* Handle the case where an LSM tree is missing obsolete files more gracefully.
KNF
Merge pull request #1304 from wiredtiger/lsm-drop-resilience

Fixup LSM drop:
whitespace
Remove unsupported schema_threads.
Merge pull request #1303 from wiredtiger/wtperf-long-transactions

Add long transaction support to wtperf.
Fix a comment.
The type returned from pthread_self() is opaque, we can't cast it to a
(void *) and print it, use the underlying WiredTiger helper routine.
/thread/t.c:49:12: error: redundant redeclaration of __wt_getopt
[-Werror=redundant-decls]
Ensure that there are switch work units for LSM trees that need them.

Two cases where switches may be needed are:
    * Failed switches that still have NEED_SWITCH set
    * If a thread is blocked waiting for a switch.

References #1305
Tweak the LSM cursor enter loop slightly.

It makes more sense to me if we push the work unit to trigger the switch then wait...
Fix the switch logic so we set WT_LSM_TREE_NEED_SWITCH when there is no primary.
Merge pull request #1308 from wiredtiger/lsm-switch-compact

Ensure that there are switch work units for LSM trees that need them.
LSM cursors need to resolve key/value buffers before calling __clsm_enter.  Otherwise, we can end up with the LSM cursor value pointing at a buffer in a closed cursor, which we then try to copy.

refs #1306
Add statistics to track open sessions and the range of transaction IDs that are pinned.
Close chunk cursors on LSM reset, so that nothing stays pinned in cache.
Forcefully discard a tree from cache if doing a forced drop.