fix PosixEnv::gettid()
fix PosixEnv::gettid()

PosixEnv::gettid() should NOT use pthread_self, since that function
returns a completely opaque identifier.  We don't know whether it's a
pointer to something, an int, etc, let alone whether it will fit in 64
bits.  Instead, return a thread-local uint64.

Change-Id: I44516bb7d76106d21391fdb3ae3b11a9321d787d
Signed-off-by: Colin Patrick McCabe <3142ce8a0bdba4a38073365893e6b74f72d9c512@cloudera.com>
Reviewed-on: http://gerrit.ent.cloudera.com:8080/796
Tested-by: jenkins
Reviewed-by: Michael Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@cloudera.com>

andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu