precomputes Turbine trees speculatively before shreds arrive

The commit maintains number of shreds observed in each slot within a
rolling window. If the channel is empty and there are no shreds to
retransmit, it uses the idle cycles to speculatively precompute turbine tree
for slots which have received most number of shreds within the rolling
window.

ledger/src/shred.rs

@@ -276,7 +276,7 @@ pub struct ShredId(Slot, /*shred index:*/ u32, ShredType);
 
 impl ShredId {
     #[inline]
-    pub(crate) fn new(slot: Slot, index: u32, shred_type: ShredType) -> ShredId {
+    pub fn new(slot: Slot, index: u32, shred_type: ShredType) -> ShredId {
         ShredId(slot, index, shred_type)
     }
 
@@ -285,6 +285,16 @@ impl ShredId {
         self.0
     }
 
+    #[inline]
+    pub fn index(&self) -> u32 {
+        self.1
+    }
+
+    #[inline]
+    pub fn shred_type(&self) -> ShredType {
+        self.2
+    }
+
     #[inline]
     pub(crate) fn unpack(&self) -> (Slot, /*shred index:*/ u32, ShredType) {
         (self.0, self.1, self.2)

ledger/src/shred/wire.rs

@@ -112,9 +112,10 @@ pub(super) fn get_parent_offset(shred: &[u8]) -> Option<u16> {
     Some(u16::from_le_bytes(bytes))
 }
 
-// Returns DataShredHeader.flags.
+// Returns DataShredHeader.flags if the shred is data.
+// Returns Error::InvalidShredType for coding shreds.
 #[inline]
-pub(crate) fn get_flags(shred: &[u8]) -> Result<ShredFlags, Error> {
+pub fn get_flags(shred: &[u8]) -> Result<ShredFlags, Error> {
     match get_shred_type(shred)? {
         ShredType::Code => Err(Error::InvalidShredType),
         ShredType::Data => {