Add MmapFlags option for MAP_POPULATE (unix)

This adds MmapFlags to DB.Options in case we need syscall.MAP_POPULATE
flag in Linux 2.6.23+ to do the sequential read-ahead, as discussed in [1].

---

[1]: https://github.com/coreos/etcd/issues/3786

bolt_unix.go

@@ -58,7 +58,7 @@ func mmap(db *DB, sz int) error {
 	}
 
 	// Map the data file to memory.
-	b, err := syscall.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_READ, syscall.MAP_SHARED)
+	b, err := syscall.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_READ, syscall.MAP_SHARED|db.MmapFlags)
 	if err != nil {
 		return err
 	}

bolt_unix_solaris.go

@@ -1,4 +1,3 @@
-
 package bolt
 
 import (
@@ -7,6 +6,7 @@ import (
 	"syscall"
 	"time"
 	"unsafe"
+
 	"golang.org/x/sys/unix"
 )
 
@@ -68,7 +68,7 @@ func mmap(db *DB, sz int) error {
 	}
 
 	// Map the data file to memory.
-	b, err := unix.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_READ, syscall.MAP_SHARED)
+	b, err := unix.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_READ, syscall.MAP_SHARED|db.MmapFlags)
 	if err != nil {
 		return err
 	}

db.go

@@ -63,6 +63,10 @@ type DB struct {
 	// https://github.com/boltdb/bolt/issues/284
 	NoGrowSync bool
 
+	// If you want to read the entire database fast, you can set MmapFlag to
+	// syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
+	MmapFlags int
+
 	// MaxBatchSize is the maximum size of a batch. Default value is
 	// copied from DefaultMaxBatchSize in Open.
 	//
@@ -136,6 +140,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 		options = DefaultOptions
 	}
 	db.NoGrowSync = options.NoGrowSync
+	db.MmapFlags = options.MmapFlags
 
 	// Set default values for later DB operations.
 	db.MaxBatchSize = DefaultMaxBatchSize
@@ -672,6 +677,9 @@ type Options struct {
 	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
 	// grab a shared lock (UNIX).
 	ReadOnly bool
+
+	// Sets the DB.MmapFlags flag before memory mapping the file.
+	MmapFlags int
 }
 
 // DefaultOptions represent the options used if nil options are passed into Open().