[ABANDON] Support for single-primary, multi-secondary processes

[riversand963]

This PR allows RocksDB to run in single-primary, multi-secondary process mode.
The writer is a regular RocksDB (e.g. an DBImpl) instance playing the role of a primary.
Multiple DBImplSecondary processes (secondaries) share the same set of SST files, MANIFEST, WAL files with the primary. Secondaries tail the MANIFEST of the primary and apply updates to their own in-memory state of the file system, e.g. VersionStorageInfo.

This PR has three components:

1. (Originally in Add PathNotFound subcode to IOError #4745). Add a 'PathNotFound' subcode to IOError to denote the failure when a secondary tries to open a file which has been deleted by the primary.

2. (Originally in Support non-blocking, retryable record reading #4602). Add TryReadRecord method to log::Reader so that caller can call TryReadRecord
   multiple times until a complete record is read. When a complete record is read,
   TryReadRecord returns true; when a record has remaining part yet to be
   read, or an error has occurred, TryReadRecord returns false. The caller can
   implement different retry policies.
   Example usage:

log::Reader reader(...);
Slice record;
std::string scratch;
while (reader.TryReadRecord(&record, &scratch)) {
  // process record
}
if (reader.reader_error_) {
 // handle error
} else {
 // consider retry
}

Note that reader_error_ is not exposed at the moment. Furthermore, we need
finer-grained error handling according to error type that is not exposed
either.

3. (Originally in Add support for MANIFEST tailing #4710 and Add unit test for DBImplSecondary #4820). Add implementation of the secondary, i.e. DBImplSecondary.
   3.1 Tail the primary's MANIFEST during recovery.
   3.2 Tail the primary's MANIFEST during normal processing by calling ReadAndApply.
   3.3 Tailing WAL will be in a future PR.

Test plan
A general check of all tests.

$make clean && make -j32 all check

Specifically, if you are interested in testing log reader related changes, run the following

$./log_test
$./log_test --gtest_filter=bool/RetriableLogTest.NonBlockingReadFullRecord/*

To test the basic functionality of DBImplSecondary, run the following

./db_secondary_test

All tests must pass.