ldb load, prefer ifsteam(/dev/stdin) to std::cin (#1207)

getline on std::cin can be very inefficient when ldb is loading large values, with high CPU usage in libc _IO_(un)getc, this is because of the performance penalty that comes from synchronizing stdio and iostream buffers.
See the reproducers and tests in #1133 .
If an ifstream on /dev/stdin is used (when available) then using ldb to load large values can be much more efficient.
I thought for ldb load, that this approach is preferable to using <cstdio> or std::ios_base::sync_with_stdio(false).
I couldn't think of a use case where ldb load would need to support reading unbuffered input, an alternative approach would be to add support for passing --input_file=/dev/stdin.
I have a CLA in place, thanks.

The CI tests were failing at the time of https://github.com/facebook/rocksdb/pull/1156, so this change and PR will supersede it.

tools/ldb_cmd.cc

@@ -39,6 +39,7 @@
 #include <sstream>
 #include <stdexcept>
 #include <string>
+#include <fstream>
 
 namespace rocksdb {
 
@@ -835,7 +836,10 @@ void DBLoaderCommand::DoCommand() {
 
   int bad_lines = 0;
   std::string line;
-  while (getline(std::cin, line, '\n')) {
+  // prefer ifstream getline performance vs that from std::cin istream
+  std::ifstream ifs_stdin("/dev/stdin");
+  std::istream* istream_p = ifs_stdin.is_open() ? &ifs_stdin : &std::cin;
+  while (getline(*istream_p, line, '\n')) {
     std::string key;
     std::string value;
     if (ParseKeyValue(line, &key, &value, is_key_hex_, is_value_hex_)) {