Made many rpc:call/4,5 calls safer if rex is down.

Created a couple of utility functions to handle do the wrapping. Most
rpc:call/4,5 that where not wrapped in a try/catch have been changed to
use the utility. Those that weren't appeared to want a crash due to
context (such as explictly matching for only the success value).
Updated dialyzer ignored warnings.
Fix hashtree:sha_test_ from timing out

EQC takes a long time, and uses gigabytes of memory to generate 1MB
random binaries, as evidenced by:

    eqc_gen:sample(eqc_gen:binary(1024 * 1024)).

We're still able to write a solid test, by ensuring that we always chunk
the binary into between 1 and 16 chunks. So instead of generating the
chunk size, we now generate the number of chunks, and derive chunk size
from that. This change allowed us to remove one level of ?FORALL
nesting, as well.
Extend hashtree eqc test timeouts

The sha_test_ and eqc_test_ test suites in the hashtree module have
had trouble with slowness and timeouts on slower test machines.  Use
spawn so each test runs in its own process and extend the timeouts to
120 seconds to avoid these spurious failures.
Merge pull request #587 from basho/bugfix/hashtree-prop_sha-timeout

Fix hashtree:sha_test_ from timing out
Add extended timeouts and eunit spawn for chashbin basic_test
Extend timeouts to 60 seconds for riak_core_security_tests
Merge pull request #589 from basho/bugfix/hashtree-test-timeouts

Extend hashtree eqc test timeouts
Merge pull request #586 from basho/bugfix/mw/safer_rpc

Made many rpc:call/4,5 calls safer if rex is down.