Introducing IO Dispatcher (#14135)

Summary:
This diff introduces the IO Dispatcher, which will be used to simplify the code path for MultiScan, while further providing a centralized place to enact policy on how MultiScan is done (i.e., limit memory usage and pinned buffers for example). Right now this diff only encapsulates the functionality done during the Prepare of MultiScan.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/14135

Reviewed By: anand1976

Differential Revision: D87837261

Pulled By: krhancoc

fbshipit-source-id: 2698910ade02bc3d182413ae07ce69fe7abb7ec5

BUCK

@@ -268,6 +268,7 @@ cpp_library_wrapper(name="rocksdb_lib", srcs=[
         "util/dynamic_bloom.cc",
         "util/file_checksum_helper.cc",
         "util/hash.cc",
+        "util/io_dispatcher_imp.cc",
         "util/murmurhash.cc",
         "util/random.cc",
         "util/rate_limiter.cc",
@@ -5214,6 +5215,12 @@ cpp_unittest_wrapper(name="interval_test",
             extra_compiler_flags=[])
 
 
+cpp_unittest_wrapper(name="io_dispatcher_test",
+            srcs=["util/io_dispatcher_test.cc"],
+            deps=[":rocksdb_test_lib"],
+            extra_compiler_flags=[])
+
+
 cpp_unittest_wrapper(name="io_posix_test",
             srcs=["env/io_posix_test.cc"],
             deps=[":rocksdb_test_lib"],