[hsync] Cache serialized block token in output stream to reduce heap consumption

hbase_rs-ccycloud-2.weichiu.root.comops.site.html

In BlockOutputStream, client serializes block token into a string for every request. This is wasteful for applications that send thousands of requests per second.

Proposal: since block token isn't supposed to change (at least within a day), cache the serialized block token and use the string every time.