Handle large writes in SendAndReset, adjust index default size

[badrishc]

Network Sends

• Handle unexpected large writes in SendAndReset by throwing an exception so that the session can end gracefully
• Support larger echo responses by splitting the response through a WriteDirectLarge wrapper

Index Defaults

• Change default main store index size to 128m (optimized for up to ~8m distinct keys)
• Change default object store index size to 16m (optimized for up to ~1m distinct keys)
• Add logger informational message to indicate what the index is optimized for

Misc

• Set CheckpointDir to LogDir (or current directory if LogDir is null) if CheckpointDir is not directly provided.

Fixes #380.