rql_test: remove previous tested database dir before reusing it

We will assign a random port to each server, and then use it to
construct a database dir in form of "run/server_${port}".

As we don't use same port number in one run, we met no issues. However,
we never delete those databases, thus we are, of course, in a high
chance that we will use a same dir we used before, which results to a
following error:

    ======================================================================
    ERROR: test_repl (__main__.TestConnection)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "connections/connection.py", line 180, in setUp
        r.db_create('test').run(conn)
      File "../../drivers/python/rethinkdb/ast.py", line 108, in run
        return c._start(self, **global_opt_args)
      File "../../drivers/python/rethinkdb/net.py", line 193, in _start
        return self._send_query(query, term, global_opt_args)
      File "../../drivers/python/rethinkdb/net.py", line 300, in _send_query
        self._check_error_response(response, term)
      File "../../drivers/python/rethinkdb/net.py", line 268, in _check_error_response
        raise RqlRuntimeError(message, term, frames)
    RqlRuntimeError: Database `test` already exists. in:
    r.db_create('test')
    ^^^^^^^^^^^^^^^^^^^

It happens because we *did* already create "test" database before.

Unconditionally removing a database directory if we are going to use
it again will fix it.

BTW, why it is worth to keep them? Can we just remove them after test?
Or, if you are considering for later check, I guess it's better to
store them at test_result dir, then it will keep the database director
being uniq?

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Added errno to crash message after bad ::rename.
Merge branch 'next' into sam_392
Bump driver versions to 1.12.0-0

OTS review by @neumino
Added a space to our precious pointer type parameter.  OTS review by @mlucy, @atnnn, @tryneus, @danielmewes.
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next
Release notes for 1.12.0

Closes #2153
Merge branch 'sam_392' into next
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next
rql_test: remove unnecessary loading of TestWithConnection class

There is no test in TestWithConnection class but only a setUp and
tearDown method.
Merge pull request #2151

Closes #2151
Merge pull request #2147 from Aleaxander/yliu-py-connect-fix

rql_test: remove previous tested database dir before reusing it
changing cache_balancer evaluation of read_ahead_ok
having dummy balancers disable read ahead at startup
Disable cross coro backtraces on OSX

OTS review by @srh and @danielmewes
test: http_admin: fix wrong pack of ack_expectations[uuid]

ack_expectations[uuid] needs an object instead of a number.

It's a long time ago change since commit 7b52a05("Fix log, refactor
get_durability, fix switch primary"), however that commit forgot to
change it on http_admin.py side, thus introduced following error
while running regress-852 test:

    Traceback (most recent call last):
      File "$RETHINKDBtest/regression/issue_852.py", line 41, in <module>
        affinities = {dc: 1}, ack_expectations = {dc: 2})
      File "$RETHINKDB/test/common/http_admin.py", line 562, in add_namespace
        info = self.do_query("POST", "/ajax/semilattice/%s_namespaces/new" % protocol, data_to_post)
      File "$RETHINKDB/test/common/http_admin.py", line 349, in do_query
        return self.do_query_specific(host, http_port, method, route, payload)
      File "$RETHINKDB/test/common/http_admin.py", line 361, in do_query_specific
        return json.loads(self.do_query_specific_plaintext(host, http_port, method, route, payload, headers))
      File "$RETHINKDB/test/common/http_admin.py", line 374, in do_query_specific_plaintext
        raise BadServerResponse(response.status, response.reason)
    http_admin.BadServerResponse: Server returned error code: 400 Bad Request

And you can see following error from rethinkdb log:

    info: HTTP request throw a schema_mismatch_exc_t with what = Expected object instead got: 2

This patch does make the "400 Bad Request" error dispear, but the
testcase still failed due to rethinkdb crash.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Merge branch '2161' into HEAD
Use libc++ to build re2 on OS X
Update release notes
Fixed trivial style problems.