sstable: unify sstable size estimation by using a single abstraction

The index block size estimation, and the sstable size estimation for data blocks
had similar code, which could be abstracted into a single type. This pr does that.

sstable/testdata/size_estimate

@@ -0,0 +1,75 @@
+# Sequence of ops which tests all of the code paths in the size_estimate type.
+
+init 1
+----
+success
+
+# Empty size should be 1
+size
+----
+1
+
+# There's a single inflight entry, so the size should be 4.
+add_inflight 4
+----
+4
+
+# After compression, entry only had a size of 3. The total size is the 3, but the
+# max estimated size yet is 4.
+entry_written 3 4 3
+----
+4
+
+# Compression ratio is 0.75 at this point. The total size is 3, and the inflight size is
+# 4, so that returned size is uint64(6.75). 
+add_inflight 5
+----
+6
+
+# We don't clear the empty size, so even after clearing a size of 1 is returned.
+clear
+----
+1
+
+# Test writing multiple inflight entries.
+add_inflight 4
+----
+4
+
+add_inflight 5
+----
+9
+
+# First inflight entry written. The entry didn't get compressed. The total size now is less than 9,
+# but the max estimated size should still be 9.
+entry_written 4 4 4
+----
+9
+
+# At this point, inflightSize is 13, the totalSize is 4. The compression ratio is 1. So, the returned
+# size should be 17.
+add_inflight 8
+----
+17
+
+# One entry has been written.
+num_entries
+----
+1
+
+# The inflight entry had a size of 5, but the entry added had a size of 3 because of compression/size estimation.
+# The compression ratio is 0.77 at this point. The inflightSize is 8. The true size is 13.16, but the maxEstimatedSize
+# is returned.
+entry_written 7 5 3 
+----
+17
+
+# The inflight size is 0, and the total size is 11.
+entry_written 11 8 4
+----
+17
+
+# The compression ratio is 0.64, and the inflight size is 20, 20*0.64 = 12.8, so the total size is uint64(12.8 + 11)
+add_inflight 20
+----
+23