LMDB read-only transaction limit fix

[kmatzen]

I incorrectly assumed that you could have many concurrent read-only transactions with LMDB. This preallocates a single read-only transaction that can be reused until commit has been called.

This still leaks cursors. I need to find out whether or not those are automatically cleaned up when a transaction aborts or commits.

New test cases included.

[seanbell]

This patch seems to fix some failed database unit tests (from #1238). Without this patch, I get an error in the "dynamic" linked tests. It happens in multiple tests. One example error that happens if you use #1238 but not #1319:

[ RUN      ] DataLayerTest/1.TestReadCropTrainSequenceUnseededLMDB
I1015 19:14:43.326297 18597 test_data_layer.cpp:41] Using temporary dataset /tmp/caffe_test.8SJB3R/db
I1015 19:14:43.328063 18597 data_layer.cpp:31] Opening dataset /tmp/caffe_test.8SJB3R/db
*** Aborted at 1413414883 (unix time) try "date -d @1413414883" if you are using GNU date ***
PC: @     0x2b73849ed8c7 __pthread_mutex_lock_full
*** SIGSEGV (@0x2b7390cda058) received by PID 18597 (TID 0x2b73907bdb00) from PID 18446744071843979352; stack trace: ***
    @     0x2b73849f7cb0 (unknown)
    @     0x2b73849ed8c7 __pthread_mutex_lock_full
    @     0x2b73829502a6 mdb_txn_renew0
    @     0x2b73829512ef mdb_txn_begin
    @           0x6dd56d caffe::LmdbDataset<>::open()
    @           0x76e1f2 caffe::DataLayer<>::DataLayerSetUp()
    @           0x77f5b3 caffe::BaseDataLayer<>::LayerSetUp()
    @           0x77f5c9 caffe::BasePrefetchingDataLayer<>::LayerSetUp()
    @           0x5e21d8 caffe::DataLayerTest<>::TestReadCropTrainSequenceUnseeded()
    @           0x69266d testing::internal::HandleExceptionsInMethodIfSupported<>()
    @           0x68a441 testing::Test::Run()
    @           0x68a526 testing::TestInfo::Run()
    @           0x68a667 testing::TestCase::Run()
    @           0x68a9be testing::internal::UnitTestImpl::RunAllTests()
    @           0x6921ed testing::internal::HandleExceptionsInMethodIfSupported<>()
    @           0x689a9e testing::UnitTest::Run()
    @           0x46ddd0 main
    @     0x2b7384c2676d (unknown)
    @           0x4738bd (unknown)
Segmentation fault (core dumped)
make: *** [runtest] Error 139

[shelhamer]

Merging as partial fix while the full fix and interface simplification is done according to the plan in the #1238 post-merge discussion.