[hammer] fix replay & update modes

Fixes several issues regarding time:

• no more spurious overhead of 500ms at the runner level
• however, runner clock wall factor (x1.5) is kept
• fixed overhead timeout when replaying a CRC hint (mode replay and/or mode update)

More precisely:

• large CRC times are limited to 2*T of the current strategy
• small CRC times are upgraded to 1*T of the current strategy
• constant overhead of 500ms is applied against fluctuations

Fixes #113 (closed)