Fix decode buffer pool length in forestdb_iterator

the docBufPool in forestdb_iterator needs to be in sync with
maxIndexEntrySize in index_entry.go. After the last encoding scheme change,
this seems to have gone out of sync.

index_entry.go, key buf size is
maxIndexEntrySize = MAX_SEC_KEY_BUFFER_LEN + MAX_DOCID_LEN + 2

in forestdb_iterator, it is
docBufPool = common.NewByteBufferPool(MAX_SEC_KEY_BUFFER_LEN)

docBufPool can be updated to same value as maxIndexEntrySize