Fetch: free QuickJS response body chain after reading

Fetch: free QuickJS response body chain after reading

Previously, Response.text(), json() and arrayBuffer() joined the body
chain into a new block of the engine pool and left both the chain
nodes and the joined copy in the pool.  The memory was released only
with the engine, so with js_context_reuse every response body read
accumulated in the pool, unaccounted by the context reuse memory
check.

Now, the chain is destroyed after the body is read.  For text() the
string is created with qjs_string_create_chb(), and for json() the
joined copy is freed after parsing.

The issue was introduced in b7f76b71 (0.9.1).