Compile warnings

[cogutvalera]

We should fix all compile warnings:

[ 91%] Building CXX object tests/CMakeFiles/chain_test.dir/tests/database_api_tests.cpp.o
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/tests/database_api_tests.cpp: In member function ‘void database_api_tests::get_account_limit_orders::test_method()’:
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/tests/database_api_tests.cpp:729:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0 ; i < results.size() - 1 ; ++i)
                       ^
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/tests/database_api_tests.cpp:742:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0 ; i < results.size() - 1 ; ++i)
                       ^
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/tests/database_api_tests.cpp:758:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0 ; i < results.size() - 1 ; ++i)
                       ^
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/tests/database_api_tests.cpp:781:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0 ; i < results.size() - 1 ; ++i)
                       ^
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/tests/database_api_tests.cpp:799:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0 ; i < results.size() - 1 ; ++i)

[ 99%] Building CXX object tests/CMakeFiles/es_test.dir/common/database_fixture.cpp.o
In file included from /home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/tests/common/database_fixture.cpp:30:0:
/home/valera/Projects/Blockchain/Graphene/BitShares/bitshares-core/libraries/plugins/elasticsearch/include/graphene/elasticsearch/elasticsearch_plugin.hpp:50:15: warning: ‘size_t graphene::elasticsearch::WriteCallback(void*, size_t, size_t, void*)’ defined but not used [-Wunused-function]
 static size_t WriteCallback(void *contents, size_t size, size_t nmemb, void *userp)

[abitmore]

There are already pull requests for these. #1181 and #1122.